A environment-friendly utilization of flue gas desulfurization residue for removing hexavalent chromium from electroplating wastewater

A environment-friendly utilization of flue gas desulfurization residue for removing hexavalent chromium from electroplating wastewater | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article A environment-friendly utilization of flue gas desulfurization residue for removing hexavalent chromium from electroplating wastewater Dean Fang, Xueyuan Li, Xiangxin Xue This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7831507/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 18 Feb, 2026 Read the published version in Transition Metal Chemistry → Version 1 posted 7 You are reading this latest preprint version Abstract A novel environment-friendly utilization manner of the flue gas desulfurization residue from wet magnesia flue gas desulfurization process and semi-dry calcium flue gas desulfurization process was proposed. In this article, the wet magnesia flue gas desulfurization residue and the semi-dry calcium flue gas desulfurization residue were respectively regarded as a high price-performance water treatment agent to detoxify the hexavalent chromium in the electroplating wastewater. The main technological mechanism and optimal parameters of the hexavalent chromium removal process were investigated. When the detoxification process was performed by using a residue from wet magnesia flue gas desulfurization process, the residual pollutant concentration in treated wastewater under the optimal experimental parameters was as follow: the hexavalent chromium residue decreased to 0.035 mg/L, total chromium residue decreased to 0.08 mg/L and magnesium residue was 73.5 mg/L. When the detoxification process was performed by using a residue from semi-dry calcium flue gas desulfurization process, the residual pollutant concentration in treated wastewater under the optimal experimental parameters was as follow: the hexavalent chromium residue decreased to 0.021 mg/L, total chromium residue decreased to 0.085 mg/L and calcium residue was 227 mg/L. The present work not only provided a new resource utilization approach of desulfurization residue from wet magnesia desulfurization technology and semi-dry calcium desulfurization technology, but also provided a high price-performance, efficient and environmentally friendly detoxification process for the electroplating wastewater which give attention to economic benefit and environmental factors. wet magnesia flue gas desulfurization residue semi-dry calcium flue gas desulfurization residue hexavalent chromium electroplating wastewater resource utilization Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 1. Introduction As one of the widespread use corrosion protection techniques, the electroplating was most commonly used for metal finishing in order to prevent corrosion of metal products and equipment.[ 1 – 3 ] While, the electroplating process generated large amounts of hazardous wastewater arising from the various complex steps of electroplating.[ 3 – 5 ] In fact, the electroplating wastewater contained multitudinous heavy metal pollution, and the pollution concentration was well above the allowable emission limits. In these heavy metal pollution, chromium(Cr) was toxic even at a low concentration, which was harmful to the human health as well as to the ecological environment, especially the hexavalent chromium (Cr(VI)). The hexavalent chromium salts was used widely in chrome plating technology might be attributed to its higher water-soluble and spontaneous reduction to trivalent chromium(Cr(III)), which was plated on the metal surface. The main source of chromium containing electroplating wastewater was waste plating solution and electroplate component washing wastewater which contain high concentrations of chromium(VI) with low pH.[ 6 – 8 ] Because of its hypertoxic, carcinogenic and bio-accumulation, the hexavalent chromium must be carefully removed or reduced to trivalent chromium in order to meet the permissible emission limits.[ 9 – 13 ] At present, flue gas desulfurization (FGD) system is a principal pipe-end treatment method to reduce SO 2 due to the mature craft and wide application.[ 14 – 18 ] Over the past couple of decades, the wet limestone-gypsum FGD system was regard as a main current technology which occupied about 90 percent of the flue gas desulfurization actual project cases in China. In recent years, some emerging FGD technology such as the wet magnesia FGD technology and the semi-dry calcium FGD technology has gradually become a hot desulfurization solution for the industrial boilers, smelting sintering plant or small and medium sized thermal power plant in China.[ 19 – 24 ] The wet magnesium oxide FGD technology had a similar technical principle with the wet lime-gypsum method. In this technology, the magnesium oxide slurry was used as SO 2 absorbent after maturation treatment. The dusts-removing flue gas must be sufficient contacted with magnesium oxide slurry in the spraying scrubber tower. During the process, the SO 2 could be trapped and absorbed by magnesium hydrate slurry droplets, thereby removing SO 2 from the flue gas. Finally, the SO 2 was fixed into the desulfurization slurry in the form of insoluble sediment. After the absorbed slurry approached absorption saturation point, the insoluble sediment could be dehydrated in pressure filtration equipment, and the essential component of the filter residue or FGD residue was magnesium sulfite hydrate.[ 15 , 21 , 22 – 25 ] Compared to traditional process, the wet magnesia FGD technology was characterized by low equipment investment, high desulfurization efficiency, simplification flow sheet, small area occupied, reliable operation, wide material sources and popular price of the calcined magnesia absorbent.[ 25 – 30 ] Since the 1980s, the semi-dry FGD technology has been matured and adopted commercially all over the world. As a typical semi-dry FGD technology, the spray dry scrubber process could be used to effectively removed SO 2 from exhaust gas which has been developed as an alternative to wet scrubber.[ 18 , 23 – 24 ] Compared to the mainstream wet limestone-gypsum FGD technology, the main process mechanism of semi-dry calcium-based FGD technology was similar. In the process, lime slurry was widely used as a SO 2 absorbent by maturation treatment after calcium oxide mixing with water. Subsequently, the lime slurry sprayed into a spray dry scrubber to form lime droplets. In absorption operating unit, the lime droplets synchronously evaporated and reacted with SO 2 to form calcium sulfite particle in older to removed SO 2 from flue gas. Then, the saturated state sorbent could be exhausted with the flue gas airflow into the ash hopper. Hence, the residue from semi-dry FGD process was a dry powdered compound contained calcium sulfite, calcarea carbonica or unreacted absorbent.[ 30 – 35 ] Compared to the mainstream wet limestone-gypsum process, the advantages of semi-dry calcium FGD technology were as follow: high desulfurization efficiency, simplification flow sheet, small area occupied, low operating cost and no wastewater discharge. Nowadays, the study on FGD residue disposal derived from the wet magnesia FGD technology and the semi-dry calcium FGD technology has caused a wide concern. The wet magnesia FGD residue disposal technologies could be grossly divided into three methods: forced oxidation discharge, recovery of magnesium sulfate, thermolysis regeneration.As the main component of wet magnesia FGD residue, MgSO 3 could be converted to MgSO 4 by forced aeration-oxidation to realized the harmlessness, and MgSO 4 also could be recovered by purification and crystallization process. Besides, regeneration of activated magnesia and vitriol from FGD residue by thermal decomposition was also an attractive methods.[ 21 – 24 ] The residue from semi-dry calcium FGD technology was mainly used as a low-cost raw material of building materials such as autoclaved brick or cement. Besides, semi-dry calcium FGD residue also could be used to regenerated the active calcium oxide by thermal decomposition. Even though the wet magnesia FGD technology and the semi-dry calcium FGD technology have been successfully used in China, the by-product residue were lower commercial value than the desulfurization gypsum arise from the traditional wet limestone-gypsum FGD process due to low-value of recycled product, high equipment investment and potential secondary pollution. Under these circumstances, the overwhelming majority of FGD residue actually has been regarded as a useless solid waste without reasonable and prompt disposition. Therefore, a low-cost, reliable and environmentally friendly resourceful utilization project of the desulfurization residue from wet magnesia FGD process and the semi-dry calcium FGD process should be explored to break the ice.[ 21 – 22 , 30 – 35 ] Today, a new resourceful utilization solution obedience to the conception of “waste control by waste” is proposed. In this project, whether wet magnesia FGD residue or the semi-dry calcium FGD residue, it is could be used as a good antidote to remove hexavalent chromium from electroplating industrial wastewater. In this study, a new environmental-friendly resourceful utilization manner for removing hexavalent chromium from electroplating industrial wastewater by using a residue from wet magnesia FGD process or semi-dry calcium FGD process was fully proven. Compared to the mainstream treatment methods of electroplating industrial wastewater and FGD residue, the new technology is a low-cost, efficient and environmental-friendly method which give attention to economic benefit and environmental factors. 2. Materials and methods 2.1 Desulfurization residue In this work, the residue from wet magnesia FGD process was provided by a fossil-fuel power station of Datang Lubei Power Generation Co. Ltd, and the semi-dry calcium FGD residue was provided by a sintering plant of Chengde Steel Company. The mian chemical components of the two flue gas desulfurization residue mentioned above were as shown in Table 1 and Table 2 . Table 1 chemical component of wet magnesia FGD residue (wt%) MgO SO 3 2- SO 4 2- SiO 2 CaO Cl CO 3 2- TFe 25.08 38.67 3.94 4.27 1.47 0.72 2.51 0.19 Table 2 chemical component of semi-dry calcium FGD residue (wt%) CaO SO 3 2- SO 4 2- SiO 2 MgO Cl CO 3 2- TFe 46.21 33.5 1.82 0.94 1.45 1.5 14.75 0.23 The XRD analysis of the residues respectively from the wet magnesia FGD process and the semi-dry calcium FGD process were exhibited in Fig. 1 (a)(b), which was to reveal the mineral phase structure. Thus the main existing forms of the pollutant elements mentioned above in FGD residue could be well described. According to the marked characteristic peak of the FGD residue in Fig. 1 (a), MgSO 3 ·3H 2 O, MgCO 3 , Mg(OH) 2 and MgSO 4 were the main ingredient of the residue from the wet magnesia FGD process. According to the marked characteristic peak of the FGD residue in Fig. 1 (b), CaSO 3 ·0.5H 2 O, CaCO 3 , Ca(OH) 2 and CaSO 4 ·0.5H 2 O were the main ingredient of the residue from the semi-dry calcium FGD process. 2.2 Electroplating industrial wastewater The wastewater discharged from electroplating plant was provided by Shenyang Aircraft Corporation. The electroplating wastewater containing high concentration hazardous heavy metal pollution. The hexavalent chromium(Cr(VI)) up to 45 mg/L, total chromium (TCr) up to 51 mg/L and pH = 5.0. 2.3 Chemicals A series of chemical reagents needed for the present experiment were as follow: Potassium dichromate (AR), sulfuric acid (AR), diphenylcarbazide (AR), sodium chloride (AR), sodium hydroxide (AR) and acetone (AR). The chemical reagents mentioned above were obtained by Shenyang Chemical Reagent Co., Ltd. 2.3 Analysis and detection The pH of reaction system was detected by electrode method(HJ 1147–2020). The concentration of hexavalent chromium was monitored by 1,5-diphenylcarbazide spectrophotometric method (GB 7467-87). The concentration of total chromium was monitored by flame atomic absorption spectrometry method (HJ 757–2015). The mineral phase structure of the residue from the two flue gas desulfurization processes and the coprecipitation from the wastewater was identified by X-ray diffractometer (Philips PW3040/60). The samples must be dried and grinded into powder (the particle size is about 45µm). Subsequently, the sample powder should be placed on the sample table and scraped flat with glass sheet. The detection parameters are as follow: the Cu Ka-radiation incident wavelength was λ = 1.54056 Å, the sweep rate was 6°/min and the sweep rang was 5° ~ 90°. The scanning electron microscope and energy dispersive spectrometer (SIGMA 500) were used to studied the morphology and elemental component of the precipitation. 2.4 Experimental procedure In this work, a series of experiments were operated in the 250 mL becherglas with magnetic heating stirrer at 25℃. The dilute sodium hydroxide solution(0.5 mol/L) and dilute sulphuric acid (1.0 mol/L) were used to maintain the pH of reaction system. In the first place a moderate dose of wet magnesia flue gas desulfurization residue or semi-dry calcium flue gas desulfurization residue should be mixed with 200 mL electroplating wastewater in becherglas, stirring constantly and control the pH of reaction solutions at acidic conditions until the reduction was accomplished. Subsequently, the chromium in the electroplating wastewater could be removed in form of the difficult-soluble precipitation. Finally, the precipitation should be dried at 80℃ for further analyzed. 3. Results and discussion 3.1 The major principles and feasibility analysis of the pollutant removal process In the electroplating wastewater, the main existent valence of chromium ions is hexavalent. As is known, the chromium concentration and pH value in solution was the main control factors of the chromium species existent forms. Therefore, the existent forms distribution characteristic of the chromium(VI) in the wastewater could be calculated and analyzed at actual hexavalent chromium(VI) concentration condition based on aqueous chemical equilibria software Visual MINTEQ. The species existing forms distribution of chromium(VI) were investigated in the pH scope from pH = 0 to pH = 14.0. As was shown in Fig. 2 , the main existent forms of hexavalent chromium ions in electroplating wastewater were HCrO 4 − at the initial pH = 5.0.[ 21 , 22 , 30 ] The Pourbaix diagram of Cr-S-Mg-H 2 O system and Cr-S-Ca-H 2 O system were shown in Fig. 3 . According to the electric potential and pH analysis result of Cr-S-Mg-H 2 O complex system, the redox potentials of HCrO 4 − was higher than HSO 3 − within the scope of investigation. Therefore, the HCrO 4 − could be reduced to the Cr 3+ under a reductive condition caused by HSO 3 − , which was released from the wet magnesia FGD residual in acidic condition, as shown in Eq. (1–2). It's worth noting that the suitably low-pH was beneficial to reduction of chromium(VI) due to the H + took part in the reduction process. However, the SO 2 secondary pollution would be released from the wet magnesia FGD residual in excessive acidic condition (pH < 2.0). Similarly, the redox potentials of HCrO 4 − was also higher than CaSO 3 ·0.5H 2 O within the scope of investigation in the electric potential and pH analysis result of Cr-S-Ca-H 2 O complex system. So, the HCrO 4 − also could be reduced to the Cr 3+ by CaSO 3 ·0.5H 2 O from the semi-dry calcium FGD residue in acidic condition, as shown in Eq. (3). Subsequently, the Cr 3 + in the wastewater could be removed by forming the insoluble chromium hydroxide precipitation under alkalescence condition, as shown in Eq. (4). Besides, the residual Mg 2+ and Ca 2+ in the wastewater also could be removed along with the Cr 3+ by forming coprecipitation, as shown in Eq. (5–6).[ 36 – 42 ] According to theoretical analysis above, the novel electroplating wastewater treatment method based on the classic chemical precipitation process by a residue from flue gas desulfurization process as a high price-performance water treatment agent was proved to be feasible in theory, and chromium(VI) and total chromium could be removed through reductive stage and precipitation stage respectively.The optimal technological parameters should be determined in a series of control variate experiments to ensure the technology is practical. MgSO 3 ·3H 2 O + H + → Mg 2+ + HSO 3 − + 3H 2 O (1) 2HCrO + 3HSO + 5H → 2Cr + 3SO + 5HO (2) 2HCrO + 3CaSO·0.5HO + 8H → 2Cr + 3CaSO·2HO + 0.5HO (3) Cr 3+ + 3OH − → Cr(OH) 3 ↓ (4) Mg 2+ + 2OH − → Mg(OH) 2 ↓ (5) Ca 2+ + 2OH − → Ca(OH) 2 ↓ (6) 3.2 The optimum process parameter of pH in reductive stage According to the chromium(VI) reductive mechanism analysis mentioned above, the pH value of reaction system pH is a kernel parameter to ensure the chromium(VI) could be reduced efficiently in a short time. Therefore, the optimum process parameter of pH should be explored in the first experiment stage under the following conditions: the reduction pH was in the scope of 1.5 to 3.5, the dose of wet magnesia FGD residue was 0.1 g, the dose of semi-dry calcium FGD residue was 0.13 g, reductive time was 15.0 min. As exhibited in Fig. 4 , the reducing efficiency of chromium(VI) was bound up with pH. As exhibited in Fig. 4 (a), within the scope of investigation, the reduction efficiency of chromium(VI) reduced from 99.83% to 78.2%, and the residual chromium(VI) increased from 0.075 mg/L to 9.81 mg/L. As exhibited in Fig. 4 (b), within the scope of investigation, the reduction efficiency of chromium(VI) reduced from 99.97% to 84.22%, and the residual chromium(VI) increased from 0.013 mg/L to 7.1 mg/L. It is obviously that the low-pH value have played active roles in improving the reduction efficiency of chromium(VI) by using FGD residue as a reductant. According to the emission standard of pollutants of electroplating (GB21900-2008), the chromium(VI) and total chromium should be decreased to 0.2 mg/L and 1.0 mg/L in the discharge water. So, the optimum reduction process parameter of pH was pH = 2.5. 3.3 The optimum process parameter of residue dose in reductive stage In this section, the FGD residue was regarded as a high price-performance substitute for mainstream water treatment reductant, so the dose of reducing agent has play a significant role in the redox process between chromium(VI) and FGD residue. In this work, the optimum process parameter of residue dose in reductive stage was explored in 200 mL wastewater under the following conditions: the wet magnesia FGD residue dose was in the scope of 0.08 g to 0.13 g, and semi-dry calcium FGD residue dose was in the scope of 0.1 g to 0.15 g, the optimal parameter of reduction pH should be controlled at 2.5, reductive time was 15.0 min. As exhibited in Fig. 5 , there is a indivisible link between the dose of FGD residue and the reducing degree of chromium(VI). With the dose growth of wet magnesia FGD residue from 0.08 g to 0.13 g, the reductive efficiency of chromium(VI) has an increase from 90.93% to 99.97%, and the residual chromium(VI) reduced obviously from 4.08 mg/L to 0.01 mg/L. Similar to above, with the dose growth of semi-dry calcium FGD residue from 0.1 g to 0.15 g, the reduction efficiency of chromium(VI) also has an increase from 85.44% to 99.96%, and the residual chromium(VI) reduced obviously from 6.55 mg/L to 0.015 mg/L. Once the FGD residue dose reached a certain point, the reducing efficiency of chromium(VI) also tended to be stable and the residual chromium(VI) concentration were well below emission limits (0.5 mg/L). Consequently, the optimum process parameter of residue dose respectively from wet magnesia FGD process and semi-dry calcium FGD process were 0.11 g (0.55 g/L) and 0.13 g (0.65 g/L). 3.4 The optimum process parameter of reductive time This work was to explored the optimum process parameter of reductive time in chromium(VI) reductive stage under the following conditions: reactive time was limited to 25.0 min, the optimal parameter of reduction pH should be control at 2.5, the optimum process parameter of residue dose respectively from wet magnesia FGD process and semi-dry calcium FGD process should be controlled at 0.11 g and 0.13 g per 200 mL wastewater. As exhibited in Fig. 6 (a) and Fig. 6 (b), within the scope of investigation, the residual chromium(VI) respectively reduced from initial 45mg/L to 0.031 mg/L and 0.021 mg/L. From the above, the chromium(VI) could be reduced to chromium(III) by using the residue from wet magnesia or semi-dry calcium flue gas desulfurization process as a reductant, furthermore the reducing efficiency of chromium(VI) also tended to be stable no more than 15.0 min. So, these two kinds of residue could be chosen as the high price-performance and environment-friendly substitute of mainstream water treatment reductant, such as Na 2 S 2 O 5 or FeSO 4 . Consequently, the optimum process parameter of reductive time was 15.0 min. 3.5 The optimum process parameter of pH in precipitation stage The pH was an important parameter for total chromium removal in form of a hydroxide precipitate. This section was to explored the optimum process parameter of pH in precipitation stage under the following conditions: the precipitation pH was in the scope of 6.5 to 8.5, the reaction time was 15.0 min. As exhibited in Fig. 7 (a), within the scope of investigation, the residual total chromium reduced from 3.35 mg/L to 0.04 mg/L, and the Mg 2+ synchronously reduced from 92.2 mg/L to 60.3 mg/L. As exhibited in Fig. 7 (b), within the scope of investigation, the residual total chromium also reduced from 2.24 mg/L to 0.062 mg/L, and the Ca 2+ also reduced from 242.6 mg/L to 205 mg/L. From the above, the high-pH condition was conducive to reduce the total chromium by forming the hydroxide precipitate. In practice, the residual concentration of total chromium should be controlled below 0.2 mg/L, which avoided the secondary pollution of hexavalent chromium and ensured safety and reliability. Consequently, the optimum process parameter of pH in precipitation stage was 7.5. 3.6 The optimum process parameter of reaction time in precipitation stage This section was to explored the optimum process parameter of reaction time in precipitation stage under the following conditions: the reaction time was limited to 25.0 min, the optimum process parameter of pH in precipitation stage should be controlled at 7.5. As exhibited in Fig. 8 (a) and Fig. 8 (b), within the scope of investigation, whether we used wet maguisia FGD residue or semi-dry calcium FGD residue, the residual concentration of total chromium tended to be optimal state and remained stable no more than 15.0 min. So, the optimum process parameter of reaction time in precipitation stage was 15.0 min. Finally, the residual pollutions in the treated wastewater were as follow: when the residue from wet maguisia FGD process was chosen as a reductant, the residual ρ Cr(VI) = 0.035 mg/L, ρ TCr = 0.08 mg/L and ρ Mg 2+ = 73.5 mg/L; when the residue from semi-dry calcium FGD process was chosen as a reductant, the residual ρ Cr(VI) = 0.021 mg/L, ρ TCr = 0.085 mg/L and ρ Ca 2+ = 227 mg/L。 3.7 Characterization analysis of the sediment As exhibited in Fig. 9 , the sediment from the treatment craft by using the wet magnesia FGD residue or semi-dry calcium FGD residue as a reductant were irregular amorphous agglomerates, and the main chemical component (mass fraction was calculated by oxide) of the coprecipitation was as exhibited in Table 3 and Table 4 . Table 3 chemical component of sediment (wet magnesia FGD residue, wt%) Cr 2 O 3 MgO SiO 2 TFe CaO Al 2 O 3 S 30.31 20.64 2.64 0.38 0.69 0.45 1.46 Table 4 chemical component of sediment (semi-dry calcium FGD residue, wt%) Cr 2 O 3 MgO SiO 2 TFe CaO Al 2 O 3 S 34.97 1.36 1.93 0.75 10.81 1.89 4.81 4 Conclusions In this article, a original environment-friendly utilization manner of the FGD residue from wet magnesia FGD process or semi-dry calcium FGD process for removing the hexavalent chromium from electroplating wastewater was proved to be entirely feasible and effective. When the residue from wet magnesia FGD process was used as a reductant, the optimal technological parameters were as follow: reductive pH = 2.5, FGD residue dose was 0.11 (0.55 g/L), reductive time was 15.0 min, total chromium removal pH = 7.5 and total chromium removal time was 15.0 min. Under the optimal experimental parameters, the residual pollutant concentration in treated wastewater was as follow: ρ Cr(VI) = 0.035 mg/L, ρ TCr = 0.08 mg/L and ρ Mg 2+ = 73.5 mg/L. When the residue from semi-dry calcium FGD process was used as a reductant, the optimum technological parameters were as follow: reductive pH = 2.5, FGD residue dose was 0.13 (0.65 g/L), reductive time was 15.0 min, total chromium removal pH = 7.5 and total chromium removal time was 15.0 min. Under the optimal experimental parameters, the residual pollutant concentration in treated wastewater was as follow: ρ Cr(VI) = 0.021 mg/L, ρ TCr = 0.085 mg/L and ρ Ca 2+ = 227 mg/L. In a word, this novel research not only provides a eco-friendly resource utilization approach of the solid waste from wet magnesia FGD technology and semi-dry calcium FGD technology, but also provides a high price-performance, efficient and environmental-friendly treatment technology for the electroplating wastewater which give attention to economic benefit and environmental factors. Declarations Conflict of interest The author declares that there is no conflict of in terest. Funding National Natural Science Foundation of China (No. U1502273). Author Contribution Xueyuan Li and Xiangxin Xue wrote the main manuscript textDean Fang is mainly responsible for data analysis and experimental design. References Y Neolaka, Y Lawa, J Naat, et al (2020) A Cr(VI)-imprinted-poly (4-VP-co-EGDMA) sorbent prepared using precipitation polymerization and its application for selective adsorptive removal and solid phase extraction of Cr(VI) ions from electroplating industrial wastewater. 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Powder Technol 401:117268 D Zhou, R Wei, Y Zhu(2021) Calcium sulfate whisker one-step preparation using semi-dry flue gas desulfurization ash and directional growth control. J Clean Prod 290: 125754 M. Li, Q. Guo, L. Xing (2020) Cobalt-based metal-organic frameworks promoting magnesium sulfite oxidation with ultrahigh catalytic activity and stability J Colloid Interf Sci 559:88-95. L Guo, W Liu, X Tang(2017) Reaction kinetics of non-catalyzed jet aeration oxidation of magnesium sulfite. Chem Eng J 330:870-879 Q Li, Y Yang, L Wang (2016) Mechanism and kinetics of magnesium sulfite oxidation catalyzed by multiwalled carbon nanotube. Appl Catal B Environ 203:851-858. C Barrera, V Lugo, B Bilyeu(2012) A review of chemical, electrochemical and biological methods for aqueous Cr(VI) reduction, J Hazard Mater 223:1-12. Z Diao, X Xu, H Chen (2016) Simultaneous removal of Cr(VI) and phenol by persulfate activated with bentonite-supported nanoscale zero-valent iron: reactivity and mechanism, J Hazard Mater 316:186-193. M Barakat(2011)New trends in removing heavy metals from industrial wastewater, J Chem 4:361-377. M Gheju, I Balcu (2011) Removal of chromium from Cr(VI) polluted wastewaters by reduction with scrap iron and subsequent precipitation of resulted cations, J Hazard Mater 196:131-138. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 18 Feb, 2026 Read the published version in Transition Metal Chemistry → Version 1 posted Editorial decision: Revision requested 27 Nov, 2025 Reviews received at journal 27 Nov, 2025 Reviewers agreed at journal 28 Oct, 2025 Reviewers invited by journal 11 Oct, 2025 Editor assigned by journal 11 Oct, 2025 Submission checks completed at journal 11 Oct, 2025 First submitted to journal 10 Oct, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7831507","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":533006232,"identity":"96060b94-1697-4728-87b3-2b48bf1caeb6","order_by":0,"name":"Dean 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13:21:08","extension":"html","order_by":24,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":105999,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7831507/v1/1e92380d7e6bd558c19afa08.html"},{"id":94370122,"identity":"4b6c50e8-1eb7-4e24-a1bf-5bf075120afd","added_by":"auto","created_at":"2025-10-27 13:21:21","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":103526,"visible":true,"origin":"","legend":"\u003cp\u003eThe XRD of wet maguisia FGD residue(a) and semi-dry calcium FGD residue(b)\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7831507/v1/d105439d137f730a386fbdbe.png"},{"id":94370120,"identity":"c115b305-0908-44f3-a1f4-529ae1011743","added_by":"auto","created_at":"2025-10-27 13:21:21","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":75324,"visible":true,"origin":"","legend":"\u003cp\u003eThe existent forms distribution of chromium(VI) in aqueous solutions\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7831507/v1/60419d5d452f8d2ce9f745d4.png"},{"id":94370454,"identity":"41a7062e-924b-41fc-b010-140badbfedb8","added_by":"auto","created_at":"2025-10-27 13:21:55","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":159680,"visible":true,"origin":"","legend":"\u003cp\u003ePourbaix diagrams of the complex system(Cr-S-Mg-H\u003csub\u003e2\u003c/sub\u003eO(a) and Cr-S-Ca-H\u003csub\u003e2\u003c/sub\u003eO(b)).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7831507/v1/adb742f2f3efcd9a88b440a0.png"},{"id":94369892,"identity":"76df99e5-7040-49c9-89ca-490492a6a555","added_by":"auto","created_at":"2025-10-27 13:20:34","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":68797,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of pH value on chromium(VI) reductive stage ((a)wet magnesia FGD residue dose is 0.1 g; (b)semi-dry calcium FGD residue dose is 0.13 g, reductive time was 15.0 min)\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7831507/v1/82dc2017673436bddf39842e.png"},{"id":94369946,"identity":"d1157ef9-9fa8-4d2b-b55c-aa69a8a6143f","added_by":"auto","created_at":"2025-10-27 13:20:53","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":87288,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of FGD residue dose on chromium(VI) reductive stage (The optimal parameter of reduction pH should be controlled at 2.5, reductive time was 15.0 min)\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7831507/v1/7416ac047989e8c4bfb87df6.png"},{"id":94369993,"identity":"2a985662-8c5c-4398-8e9c-e650d53da4c2","added_by":"auto","created_at":"2025-10-27 13:20:56","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":50470,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of reactive time on chromium(VI) reductive stage (the optimal parameter of the reductive pH should be control at 2.5, (a) the wet magnesia FGD residue dose is 0.11 g; (b) semi-dry calcium FGD residue dose is 0.13 g )\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7831507/v1/6ed13aa5073229132bb42f51.png"},{"id":94370365,"identity":"5d57ac88-3757-4bd8-93c6-9388b1fc70d8","added_by":"auto","created_at":"2025-10-27 13:21:42","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":76814,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of pH in precipitation stage ( t=15.0 min.)\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-7831507/v1/cb54d04f85cd1481dce2b181.png"},{"id":94370307,"identity":"6f3b839d-df58-49b9-9f60-3c6859c9f2ea","added_by":"auto","created_at":"2025-10-27 13:21:35","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":83253,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of reaction time in precipitation stage (The the optimum process parameter of pH in precipitation stage should be controlled at 7.5)\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-7831507/v1/9f81eb3b3e9daa7451ccb9d7.png"},{"id":94369764,"identity":"f08a46ca-111a-480a-b77b-c4beb63a44e0","added_by":"auto","created_at":"2025-10-27 13:19:22","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":361463,"visible":true,"origin":"","legend":"\u003cp\u003eThe XRD(a,d), SEM(b,e) and EDS patterns(c,f) of sediment (a, b, c-wet magnesia FGD residue; d, e, f-semi-dry calcium FGD residue)\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-7831507/v1/8905534bd0f6cca2dca1a9ad.png"},{"id":103251528,"identity":"52153000-836f-4263-afc1-f3a84555bf71","added_by":"auto","created_at":"2026-02-23 16:10:28","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1724669,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7831507/v1/10b8d480-aa44-4acf-9e99-55c4c0b11a41.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"A environment-friendly utilization of flue gas desulfurization residue for removing hexavalent chromium from electroplating wastewater","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eAs one of the widespread use corrosion protection techniques, the electroplating was most commonly used for metal finishing in order to prevent corrosion of metal products and equipment.[\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] While, the electroplating process generated large amounts of hazardous wastewater arising from the various complex steps of electroplating.[\u003cspan additionalcitationids=\"CR4\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e] In fact, the electroplating wastewater contained multitudinous heavy metal pollution, and the pollution concentration was well above the allowable emission limits. In these heavy metal pollution, chromium(Cr) was toxic even at a low concentration, which was harmful to the human health as well as to the ecological environment, especially the hexavalent chromium (Cr(VI)). The hexavalent chromium salts was used widely in chrome plating technology might be attributed to its higher water-soluble and spontaneous reduction to trivalent chromium(Cr(III)), which was plated on the metal surface. The main source of chromium containing electroplating wastewater was waste plating solution and electroplate component washing wastewater which contain high concentrations of chromium(VI) with low pH.[\u003cspan additionalcitationids=\"CR7\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e] Because of its hypertoxic, carcinogenic and bio-accumulation, the hexavalent chromium must be carefully removed or reduced to trivalent chromium in order to meet the permissible emission limits.[\u003cspan additionalcitationids=\"CR10 CR11 CR12\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/p\u003e\u003cp\u003eAt present, flue gas desulfurization (FGD) system is a principal pipe-end treatment method to reduce SO\u003csub\u003e2\u003c/sub\u003e due to the mature craft and wide application.[\u003cspan additionalcitationids=\"CR15 CR16 CR17\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] Over the past couple of decades, the wet limestone-gypsum FGD system was regard as a main current technology which occupied about 90 percent of the flue gas desulfurization actual project cases in China. In recent years, some emerging FGD technology such as the wet magnesia FGD technology and the semi-dry calcium FGD technology has gradually become a hot desulfurization solution for the industrial boilers, smelting sintering plant or small and medium sized thermal power plant in China.[\u003cspan additionalcitationids=\"CR20 CR21 CR22 CR23\" citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/p\u003e\u003cp\u003eThe wet magnesium oxide FGD technology had a similar technical principle with the wet lime-gypsum method. In this technology, the magnesium oxide slurry was used as SO\u003csub\u003e2\u003c/sub\u003e absorbent after maturation treatment. The dusts-removing flue gas must be sufficient contacted with magnesium oxide slurry in the spraying scrubber tower. During the process, the SO\u003csub\u003e2\u003c/sub\u003e could be trapped and absorbed by magnesium hydrate slurry droplets, thereby removing SO\u003csub\u003e2\u003c/sub\u003e from the flue gas. Finally, the SO\u003csub\u003e2\u003c/sub\u003e was fixed into the desulfurization slurry in the form of insoluble sediment. After the absorbed slurry approached absorption saturation point, the insoluble sediment could be dehydrated in pressure filtration equipment, and the essential component of the filter residue or FGD residue was magnesium sulfite hydrate.[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan additionalcitationids=\"CR23 CR24\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e] Compared to traditional process, the wet magnesia FGD technology was characterized by low equipment investment, high desulfurization efficiency, simplification flow sheet, small area occupied, reliable operation, wide material sources and popular price of the calcined magnesia absorbent.[\u003cspan additionalcitationids=\"CR26 CR27 CR28 CR29\" citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/p\u003e\u003cp\u003eSince the 1980s, the semi-dry FGD technology has been matured and adopted commercially all over the world. As a typical semi-dry FGD technology, the spray dry scrubber process could be used to effectively removed SO\u003csub\u003e2\u003c/sub\u003e from exhaust gas which has been developed as an alternative to wet scrubber.[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e] Compared to the mainstream wet limestone-gypsum FGD technology, the main process mechanism of semi-dry calcium-based FGD technology was similar. In the process, lime slurry was widely used as a SO\u003csub\u003e2\u003c/sub\u003e absorbent by maturation treatment after calcium oxide mixing with water. Subsequently, the lime slurry sprayed into a spray dry scrubber to form lime droplets. In absorption operating unit, the lime droplets synchronously evaporated and reacted with SO\u003csub\u003e2\u003c/sub\u003e to form calcium sulfite particle in older to removed SO\u003csub\u003e2\u003c/sub\u003e from flue gas. Then, the saturated state sorbent could be exhausted with the flue gas airflow into the ash hopper. Hence, the residue from semi-dry FGD process was a dry powdered compound contained calcium sulfite, calcarea carbonica or unreacted absorbent.[\u003cspan additionalcitationids=\"CR31 CR32 CR33 CR34\" citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e] Compared to the mainstream wet limestone-gypsum process, the advantages of semi-dry calcium FGD technology were as follow: high desulfurization efficiency, simplification flow sheet, small area occupied, low operating cost and no wastewater discharge.\u003c/p\u003e\u003cp\u003eNowadays, the study on FGD residue disposal derived from the wet magnesia FGD technology and the semi-dry calcium FGD technology has caused a wide concern. The wet magnesia FGD residue disposal technologies could be grossly divided into three methods: forced oxidation discharge, recovery of magnesium sulfate, thermolysis regeneration.As the main component of wet magnesia FGD residue, MgSO\u003csub\u003e3\u003c/sub\u003e could be converted to MgSO\u003csub\u003e4\u003c/sub\u003e by forced aeration-oxidation to realized the harmlessness, and MgSO\u003csub\u003e4\u003c/sub\u003e also could be recovered by purification and crystallization process. Besides, regeneration of activated magnesia and vitriol from FGD residue by thermal decomposition was also an attractive methods.[\u003cspan additionalcitationids=\"CR22 CR23\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e] The residue from semi-dry calcium FGD technology was mainly used as a low-cost raw material of building materials such as autoclaved brick or cement. Besides, semi-dry calcium FGD residue also could be used to regenerated the active calcium oxide by thermal decomposition. Even though the wet magnesia FGD technology and the semi-dry calcium FGD technology have been successfully used in China, the by-product residue were lower commercial value than the desulfurization gypsum arise from the traditional wet limestone-gypsum FGD process due to low-value of recycled product, high equipment investment and potential secondary pollution. Under these circumstances, the overwhelming majority of FGD residue actually has been regarded as a useless solid waste without reasonable and prompt disposition. Therefore, a low-cost, reliable and environmentally friendly resourceful utilization project of the desulfurization residue from wet magnesia FGD process and the semi-dry calcium FGD process should be explored to break the ice.[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan additionalcitationids=\"CR31 CR32 CR33 CR34\" citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e] Today, a new resourceful utilization solution obedience to the conception of \u0026ldquo;waste control by waste\u0026rdquo; is proposed. In this project, whether wet magnesia FGD residue or the semi-dry calcium FGD residue, it is could be used as a good antidote to remove hexavalent chromium from electroplating industrial wastewater.\u003c/p\u003e\u003cp\u003eIn this study, a new environmental-friendly resourceful utilization manner for removing hexavalent chromium from electroplating industrial wastewater by using a residue from wet magnesia FGD process or semi-dry calcium FGD process was fully proven. Compared to the mainstream treatment methods of electroplating industrial wastewater and FGD residue, the new technology is a low-cost, efficient and environmental-friendly method which give attention to economic benefit and environmental factors.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Desulfurization residue\u003c/h2\u003e\u003cp\u003eIn this work, the residue from wet magnesia FGD process was provided by a fossil-fuel power station of Datang Lubei Power Generation Co. Ltd, and the semi-dry calcium FGD residue was provided by a sintering plant of Chengde Steel Company. The mian chemical components of the two flue gas desulfurization residue mentioned above were as shown in Table \u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and Table \u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003echemical component of wet magnesia FGD residue (wt%)\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"8\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMgO\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eSiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eCaO\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eCl\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c8\"\u003e\u003cp\u003eTFe\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e25.08\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e38.67\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e3.94\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e4.27\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1.47\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.72\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e2.51\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.19\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003echemical component of semi-dry calcium FGD residue (wt%)\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"8\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCaO\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eSiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eMgO\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eCl\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c8\"\u003e\u003cp\u003eTFe\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e46.21\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e33.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1.82\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.94\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1.45\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e1.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e14.75\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.23\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe XRD analysis of the residues respectively from the wet magnesia FGD process and the semi-dry calcium FGD process were exhibited in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a)(b), which was to reveal the mineral phase structure. Thus the main existing forms of the pollutant elements mentioned above in FGD residue could be well described. According to the marked characteristic peak of the FGD residue in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a), MgSO\u003csub\u003e3\u003c/sub\u003e\u0026middot;3H\u003csub\u003e2\u003c/sub\u003eO, MgCO\u003csub\u003e3\u003c/sub\u003e, Mg(OH)\u003csub\u003e2\u003c/sub\u003e and MgSO\u003csub\u003e4\u003c/sub\u003e were the main ingredient of the residue from the wet magnesia FGD process. According to the marked characteristic peak of the FGD residue in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b), CaSO\u003csub\u003e3\u003c/sub\u003e\u0026middot;0.5H\u003csub\u003e2\u003c/sub\u003eO, CaCO\u003csub\u003e3\u003c/sub\u003e, Ca(OH)\u003csub\u003e2\u003c/sub\u003e and CaSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;0.5H\u003csub\u003e2\u003c/sub\u003eO were the main ingredient of the residue from the semi-dry calcium FGD process.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Electroplating industrial wastewater\u003c/h2\u003e\u003cp\u003eThe wastewater discharged from electroplating plant was provided by Shenyang Aircraft Corporation. The electroplating wastewater containing high concentration hazardous heavy metal pollution. The hexavalent chromium(Cr(VI)) up to 45 mg/L, total chromium (TCr) up to 51 mg/L and pH\u0026thinsp;=\u0026thinsp;5.0.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Chemicals\u003c/h2\u003e\u003cp\u003eA series of chemical reagents needed for the present experiment were as follow: Potassium dichromate (AR), sulfuric acid (AR), diphenylcarbazide (AR), sodium chloride (AR), sodium hydroxide (AR) and acetone (AR). The chemical reagents mentioned above were obtained by Shenyang Chemical Reagent Co., Ltd.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Analysis and detection\u003c/h2\u003e\u003cp\u003eThe pH of reaction system was detected by electrode method(HJ 1147\u0026ndash;2020). The concentration of hexavalent chromium was monitored by 1,5-diphenylcarbazide spectrophotometric method (GB 7467-87). The concentration of total chromium was monitored by flame atomic absorption spectrometry method (HJ 757\u0026ndash;2015).\u003c/p\u003e\u003cp\u003eThe mineral phase structure of the residue from the two flue gas desulfurization processes and the coprecipitation from the wastewater was identified by X-ray diffractometer (Philips PW3040/60). The samples must be dried and grinded into powder (the particle size is about 45\u0026micro;m). Subsequently, the sample powder should be placed on the sample table and scraped flat with glass sheet. The detection parameters are as follow: the Cu Ka-radiation incident wavelength was λ\u0026thinsp;=\u0026thinsp;1.54056 \u0026Aring;, the sweep rate was 6\u0026deg;/min and the sweep rang was 5\u0026deg; ~ 90\u0026deg;. The scanning electron microscope and energy dispersive spectrometer (SIGMA 500) were used to studied the morphology and elemental component of the precipitation.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e2.4 Experimental procedure\u003c/h2\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003eIn this work, a series of experiments were operated in the 250 mL becherglas with magnetic heating stirrer at 25℃. The dilute sodium hydroxide solution(0.5 mol/L) and dilute sulphuric acid (1.0 mol/L) were used to maintain the pH of reaction system. In the first place a moderate dose of wet magnesia flue gas desulfurization residue or semi-dry calcium flue gas desulfurization residue should be mixed with 200 mL electroplating wastewater in becherglas, stirring constantly and control the pH of reaction solutions at acidic conditions until the reduction was accomplished. Subsequently, the chromium in the electroplating wastewater could be removed in form of the difficult-soluble precipitation. Finally, the precipitation should be dried at 80℃ for further analyzed.\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.1 The major principles and feasibility analysis of the pollutant removal process\u003c/h2\u003e\u003cp\u003eIn the electroplating wastewater, the main existent valence of chromium ions is hexavalent. As is known, the chromium concentration and pH value in solution was the main control factors of the chromium species existent forms. Therefore, the existent forms distribution characteristic of the chromium(VI) in the wastewater could be calculated and analyzed at actual hexavalent chromium(VI) concentration condition based on aqueous chemical equilibria software Visual MINTEQ. The species existing forms distribution of chromium(VI) were investigated in the pH scope from pH\u0026thinsp;=\u0026thinsp;0 to pH\u0026thinsp;=\u0026thinsp;14.0. As was shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, the main existent forms of hexavalent chromium ions in electroplating wastewater were HCrO\u003csub\u003e4\u003c/sub\u003e\u0026thinsp;\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026thinsp;at the initial pH\u0026thinsp;=\u0026thinsp;5.0.[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/p\u003e\u003cp\u003eThe Pourbaix diagram of Cr-S-Mg-H\u003csub\u003e2\u003c/sub\u003eO system and Cr-S-Ca-H\u003csub\u003e2\u003c/sub\u003eO system were shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. According to the electric potential and pH analysis result of Cr-S-Mg-H\u003csub\u003e2\u003c/sub\u003eO complex system, the redox potentials of HCrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e was higher than HSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e within the scope of investigation. Therefore, the HCrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e could be reduced to the Cr\u003csup\u003e3+\u003c/sup\u003e under a reductive condition caused by HSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, which was released from the wet magnesia FGD residual in acidic condition, as shown in Eq.\u0026nbsp;(1\u0026ndash;2). It's worth noting that the suitably low-pH was beneficial to reduction of chromium(VI) due to the H\u003csup\u003e+\u003c/sup\u003e took part in the reduction process. However, the SO\u003csub\u003e2\u003c/sub\u003e secondary pollution would be released from the wet magnesia FGD residual in excessive acidic condition (pH\u0026thinsp;\u0026lt;\u0026thinsp;2.0). Similarly, the redox potentials of HCrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e was also higher than CaSO\u003csub\u003e3\u003c/sub\u003e\u0026middot;0.5H\u003csub\u003e2\u003c/sub\u003eO within the scope of investigation in the electric potential and pH analysis result of Cr-S-Ca-H\u003csub\u003e2\u003c/sub\u003eO complex system. So, the HCrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e also could be reduced to the Cr\u003csup\u003e3+\u003c/sup\u003e by CaSO\u003csub\u003e3\u003c/sub\u003e\u0026middot;0.5H\u003csub\u003e2\u003c/sub\u003eO from the semi-dry calcium FGD residue in acidic condition, as shown in Eq.\u0026nbsp;(3). Subsequently, the Cr\u003csup\u003e3\u0026thinsp;+\u003c/sup\u003e\u0026thinsp;in the wastewater could be removed by forming the insoluble chromium hydroxide precipitation under alkalescence condition, as shown in Eq.\u0026nbsp;(4). Besides, the residual Mg\u003csup\u003e2+\u003c/sup\u003e and Ca\u003csup\u003e2+\u003c/sup\u003e in the wastewater also could be removed along with the Cr\u003csup\u003e3+\u003c/sup\u003e by forming coprecipitation, as shown in Eq.\u0026nbsp;(5\u0026ndash;6).[\u003cspan additionalcitationids=\"CR37 CR38 CR39 CR40 CR41\" citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]\u003c/p\u003e\u003cp\u003eAccording to theoretical analysis above, the novel electroplating wastewater treatment method based on the classic chemical precipitation process by a residue from flue gas desulfurization process as a high price-performance water treatment agent was proved to be feasible in theory, and chromium(VI) and total chromium could be removed through reductive stage and precipitation stage respectively.The optimal technological parameters should be determined in a series of control variate experiments to ensure the technology is practical.\u003c/p\u003e\u003cp\u003eMgSO\u003csub\u003e3\u003c/sub\u003e\u0026middot;3H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;H\u003csup\u003e+\u003c/sup\u003e \u0026rarr; Mg\u003csup\u003e2+\u003c/sup\u003e + HSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e + 3H\u003csub\u003e2\u003c/sub\u003eO (1)\u003c/p\u003e\u003c/div\u003e\n\u003cp\u003e2HCrO + 3HSO + 5H → 2Cr + 3SO + 5HO (2)\u003c/p\u003e\n\n\u003cp\u003e2HCrO + 3CaSO·0.5HO + 8H → 2Cr + 3CaSO·2HO + 0.5HO (3)\u003c/p\u003e\n\u003cp\u003eCr\u003csup\u003e3+\u003c/sup\u003e + 3OH\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026rarr; Cr(OH)\u003csub\u003e3\u003c/sub\u003e \u0026darr; (4)\u003c/p\u003e\u003cp\u003eMg\u003csup\u003e2+\u003c/sup\u003e + 2OH\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026rarr; Mg(OH)\u003csub\u003e2\u003c/sub\u003e \u0026darr; (5)\u003c/p\u003e\u003cp\u003eCa\u003csup\u003e2+\u003c/sup\u003e + 2OH\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026rarr; Ca(OH)\u003csub\u003e2\u003c/sub\u003e \u0026darr; (6)\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003e3.2 The optimum process parameter of pH in reductive stage\u003c/h2\u003e\u003cp\u003eAccording to the chromium(VI) reductive mechanism analysis mentioned above, the pH value of reaction system pH is a kernel parameter to ensure the chromium(VI) could be reduced efficiently in a short time. Therefore, the optimum process parameter of pH should be explored in the first experiment stage under the following conditions: the reduction pH was in the scope of 1.5 to 3.5, the dose of wet magnesia FGD residue was 0.1 g, the dose of semi-dry calcium FGD residue was 0.13 g, reductive time was 15.0 min.\u003c/p\u003e\u003cp\u003eAs exhibited in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, the reducing efficiency of chromium(VI) was bound up with pH. As exhibited in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a), within the scope of investigation, the reduction efficiency of chromium(VI) reduced from 99.83% to 78.2%, and the residual chromium(VI) increased from 0.075 mg/L to 9.81 mg/L. As exhibited in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b), within the scope of investigation, the reduction efficiency of chromium(VI) reduced from 99.97% to 84.22%, and the residual chromium(VI) increased from 0.013 mg/L to 7.1 mg/L. It is obviously that the low-pH value have played active roles in improving the reduction efficiency of chromium(VI) by using FGD residue as a reductant. According to the emission standard of pollutants of electroplating (GB21900-2008), the chromium(VI) and total chromium should be decreased to 0.2 mg/L and 1.0 mg/L in the discharge water. So, the optimum reduction process parameter of pH was pH\u0026thinsp;=\u0026thinsp;2.5.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\u003ch2\u003e3.3 The optimum process parameter of residue dose in reductive stage\u003c/h2\u003e\u003cp\u003eIn this section, the FGD residue was regarded as a high price-performance substitute for mainstream water treatment reductant, so the dose of reducing agent has play a significant role in the redox process between chromium(VI) and FGD residue. In this work, the optimum process parameter of residue dose in reductive stage was explored in 200 mL wastewater under the following conditions: the wet magnesia FGD residue dose was in the scope of 0.08 g to 0.13 g, and semi-dry calcium FGD residue dose was in the scope of 0.1 g to 0.15 g, the optimal parameter of reduction pH should be controlled at 2.5, reductive time was 15.0 min.\u003c/p\u003e\u003cp\u003eAs exhibited in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, there is a indivisible link between the dose of FGD residue and the reducing degree of chromium(VI). With the dose growth of wet magnesia FGD residue from 0.08 g to 0.13 g, the reductive efficiency of chromium(VI) has an increase from 90.93% to 99.97%, and the residual chromium(VI) reduced obviously from 4.08 mg/L to 0.01 mg/L. Similar to above, with the dose growth of semi-dry calcium FGD residue from 0.1 g to 0.15 g, the reduction efficiency of chromium(VI) also has an increase from 85.44% to 99.96%, and the residual chromium(VI) reduced obviously from 6.55 mg/L to 0.015 mg/L. Once the FGD residue dose reached a certain point, the reducing efficiency of chromium(VI) also tended to be stable and the residual chromium(VI) concentration were well below emission limits (0.5 mg/L). Consequently, the optimum process parameter of residue dose respectively from wet magnesia FGD process and semi-dry calcium FGD process were 0.11 g (0.55 g/L) and 0.13 g (0.65 g/L).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\u003ch2\u003e3.4 The optimum process parameter of reductive time\u003c/h2\u003e\u003cp\u003eThis work was to explored the optimum process parameter of reductive time in chromium(VI) reductive stage under the following conditions: reactive time was limited to 25.0 min, the optimal parameter of reduction pH should be control at 2.5, the optimum process parameter of residue dose respectively from wet magnesia FGD process and semi-dry calcium FGD process should be controlled at 0.11 g and 0.13 g per 200 mL wastewater.\u003c/p\u003e\u003cp\u003eAs exhibited in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a) and Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(b), within the scope of investigation, the residual chromium(VI) respectively reduced from initial 45mg/L to 0.031 mg/L and 0.021 mg/L. From the above, the chromium(VI) could be reduced to chromium(III) by using the residue from wet magnesia or semi-dry calcium flue gas desulfurization process as a reductant, furthermore the reducing efficiency of chromium(VI) also tended to be stable no more than 15.0 min. So, these two kinds of residue could be chosen as the high price-performance and environment-friendly substitute of mainstream water treatment reductant, such as Na\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e or FeSO\u003csub\u003e4\u003c/sub\u003e. Consequently, the optimum process parameter of reductive time was 15.0 min.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\u003ch2\u003e3.5 The optimum process parameter of pH in precipitation stage\u003c/h2\u003e\u003cp\u003eThe pH was an important parameter for total chromium removal in form of a hydroxide precipitate. This section was to explored the optimum process parameter of pH in precipitation stage under the following conditions: the precipitation pH was in the scope of 6.5 to 8.5, the reaction time was 15.0 min.\u003c/p\u003e\u003cp\u003eAs exhibited in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(a), within the scope of investigation, the residual total chromium reduced from 3.35 mg/L to 0.04 mg/L, and the Mg\u003csup\u003e2+\u003c/sup\u003e synchronously reduced from 92.2 mg/L to 60.3 mg/L. As exhibited in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(b), within the scope of investigation, the residual total chromium also reduced from 2.24 mg/L to 0.062 mg/L, and the Ca\u003csup\u003e2+\u003c/sup\u003e also reduced from 242.6 mg/L to 205 mg/L.\u003c/p\u003e\u003cp\u003eFrom the above, the high-pH condition was conducive to reduce the total chromium by forming the hydroxide precipitate. In practice, the residual concentration of total chromium should be controlled below 0.2 mg/L, which avoided the secondary pollution of hexavalent chromium and ensured safety and reliability. Consequently, the optimum process parameter of pH in precipitation stage was 7.5.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\u003ch2\u003e3.6 The optimum process parameter of reaction time in precipitation stage\u003c/h2\u003e\u003cp\u003eThis section was to explored the optimum process parameter of reaction time in precipitation stage under the following conditions: the reaction time was limited to 25.0 min, the optimum process parameter of pH in precipitation stage should be controlled at 7.5.\u003c/p\u003e\u003cp\u003eAs exhibited in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(a) and Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(b), within the scope of investigation, whether we used wet maguisia FGD residue or semi-dry calcium FGD residue, the residual concentration of total chromium tended to be optimal state and remained stable no more than 15.0 min. So, the optimum process parameter of reaction time in precipitation stage was 15.0 min.\u003c/p\u003e\u003cp\u003eFinally, the residual pollutions in the treated wastewater were as follow: when the residue from wet maguisia FGD process was chosen as a reductant, the residual ρ\u003csub\u003eCr(VI)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.035 mg/L, ρ\u003csub\u003eTCr\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.08 mg/L and ρ\u003csub\u003eMg\u003c/sub\u003e\u003csup\u003e2+\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;73.5 mg/L; when the residue from semi-dry calcium FGD process was chosen as a reductant, the residual ρ\u003csub\u003eCr(VI)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.021 mg/L, ρ\u003csub\u003eTCr\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.085 mg/L and ρ\u003csub\u003eCa\u003c/sub\u003e\u003csup\u003e2+\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;227 mg/L。\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec17\" class=\"Section2\"\u003e\u003ch2\u003e3.7 Characterization analysis of the sediment\u003c/h2\u003e\u003cp\u003eAs exhibited in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e, the sediment from the treatment craft by using the wet magnesia FGD residue or semi-dry calcium FGD residue as a reductant were irregular amorphous agglomerates, and the main chemical component (mass fraction was calculated by oxide) of the coprecipitation was as exhibited in Table \u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and Table \u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003echemical component of sediment (wet magnesia FGD residue, wt%)\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"7\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eMgO\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eTFe\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eCaO\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eS\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e30.31\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e20.64\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2.64\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.38\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.69\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.45\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e1.46\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003echemical component of sediment (semi-dry calcium FGD residue, wt%)\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"7\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eMgO\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eTFe\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eCaO\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eS\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e34.97\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1.36\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1.93\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.75\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e10.81\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e1.89\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e4.81\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"4 Conclusions","content":"\u003cp\u003eIn this article, a original environment-friendly utilization manner of the FGD residue from wet magnesia FGD process or semi-dry calcium FGD process for removing the hexavalent chromium from electroplating wastewater was proved to be entirely feasible and effective. When the residue from wet magnesia FGD process was used as a reductant, the optimal technological parameters were as follow: reductive pH\u0026thinsp;=\u0026thinsp;2.5, FGD residue dose was 0.11 (0.55 g/L), reductive time was 15.0 min, total chromium removal pH\u0026thinsp;=\u0026thinsp;7.5 and total chromium removal time was 15.0 min. Under the optimal experimental parameters, the residual pollutant concentration in treated wastewater was as follow: ρ\u003csub\u003eCr(VI)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.035 mg/L, ρ\u003csub\u003eTCr\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.08 mg/L and ρ\u003csub\u003eMg\u003c/sub\u003e\u003csup\u003e2+\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;73.5 mg/L. When the residue from semi-dry calcium FGD process was used as a reductant, the optimum technological parameters were as follow: reductive pH\u0026thinsp;=\u0026thinsp;2.5, FGD residue dose was 0.13 (0.65 g/L), reductive time was 15.0 min, total chromium removal pH\u0026thinsp;=\u0026thinsp;7.5 and total chromium removal time was 15.0 min. Under the optimal experimental parameters, the residual pollutant concentration in treated wastewater was as follow: ρ\u003csub\u003eCr(VI)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.021 mg/L, ρ\u003csub\u003eTCr\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.085 mg/L and ρ\u003csub\u003eCa\u003c/sub\u003e\u003csup\u003e2+\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;227 mg/L.\u003c/p\u003e\u003cp\u003eIn a word, this novel research not only provides a eco-friendly resource utilization approach of the solid waste from wet magnesia FGD technology and semi-dry calcium FGD technology, but also provides a high price-performance, efficient and environmental-friendly treatment technology for the electroplating wastewater which give attention to economic benefit and environmental factors.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eConflict of interest The author declares that there is no conflict of in terest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNational Natural Science Foundation of China (No. U1502273).\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eXueyuan Li and Xiangxin Xue wrote the main manuscript textDean Fang is mainly responsible for data analysis and experimental design.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eY Neolaka, Y Lawa, J Naat, et al (2020) A Cr(VI)-imprinted-poly (4-VP-co-EGDMA) sorbent prepared using precipitation polymerization and its application for selective adsorptive removal and solid phase extraction of Cr(VI) ions from electroplating industrial wastewater. 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Appl Catal B Environ 203:851-858. \u003c/li\u003e\n\u003cli\u003eC Barrera, V Lugo, B Bilyeu(2012) A review of chemical, electrochemical and biological methods for aqueous Cr(VI) reduction, J Hazard Mater 223:1-12. \u003c/li\u003e\n\u003cli\u003eZ Diao, X Xu, H Chen (2016) Simultaneous removal of Cr(VI) and phenol by persulfate activated with bentonite-supported nanoscale zero-valent iron: reactivity and mechanism, J Hazard Mater 316:186-193.\u003c/li\u003e\n\u003cli\u003eM Barakat(2011)New trends in removing heavy metals from industrial wastewater, J Chem 4:361-377.\u003c/li\u003e\n\u003cli\u003eM Gheju, I Balcu (2011) Removal of chromium from Cr(VI) polluted wastewaters by reduction with scrap iron and subsequent precipitation of resulted cations, J Hazard Mater 196:131-138. \u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"transition-metal-chemistry","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"tmch","sideBox":"Learn more about [Transition Metal Chemistry](http://link.springer.com/journal/11243)","snPcode":"11243","submissionUrl":"https://submission.nature.com/new-submission/11243/3","title":"Transition Metal Chemistry","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"wet magnesia flue gas desulfurization residue, semi-dry calcium flue gas desulfurization residue, hexavalent chromium, electroplating wastewater, resource utilization","lastPublishedDoi":"10.21203/rs.3.rs-7831507/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7831507/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eA novel environment-friendly utilization manner of the flue gas desulfurization residue from wet magnesia flue gas desulfurization process and semi-dry calcium flue gas desulfurization process was proposed. In this article, the wet magnesia flue gas desulfurization residue and the semi-dry calcium flue gas desulfurization residue were respectively regarded as a high price-performance water treatment agent to detoxify the hexavalent chromium in the electroplating wastewater. The main technological mechanism and optimal parameters of the hexavalent chromium removal process were investigated. When the detoxification process was performed by using a residue from wet magnesia flue gas desulfurization process, the residual pollutant concentration in treated wastewater under the optimal experimental parameters was as follow: the hexavalent chromium residue decreased to 0.035 mg/L, total chromium residue decreased to 0.08 mg/L and magnesium residue was 73.5 mg/L. When the detoxification process was performed by using a residue from semi-dry calcium flue gas desulfurization process, the residual pollutant concentration in treated wastewater under the optimal experimental parameters was as follow: the hexavalent chromium residue decreased to 0.021 mg/L, total chromium residue decreased to 0.085 mg/L and calcium residue was 227 mg/L. The present work not only provided a new resource utilization approach of desulfurization residue from wet magnesia desulfurization technology and semi-dry calcium desulfurization technology, but also provided a high price-performance, efficient and environmentally friendly detoxification process for the electroplating wastewater which give attention to economic benefit and environmental factors.\u003c/p\u003e","manuscriptTitle":"A environment-friendly utilization of flue gas desulfurization residue for removing hexavalent chromium from electroplating wastewater","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-24 23:56:39","doi":"10.21203/rs.3.rs-7831507/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-11-27T09:01:38+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-11-27T08:08:22+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"296590772019386362733732701058260820054","date":"2025-10-28T11:30:35+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-10-11T18:53:01+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-10-11T18:43:58+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-10-11T10:30:25+00:00","index":"","fulltext":""},{"type":"submitted","content":"Transition Metal Chemistry","date":"2025-10-11T03:57:47+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"transition-metal-chemistry","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"tmch","sideBox":"Learn more about [Transition Metal Chemistry](http://link.springer.com/journal/11243)","snPcode":"11243","submissionUrl":"https://submission.nature.com/new-submission/11243/3","title":"Transition Metal Chemistry","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"b07a48e4-2940-435b-bfe9-258259b91144","owner":[],"postedDate":"October 24th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-02-23T16:07:01+00:00","versionOfRecord":{"articleIdentity":"rs-7831507","link":"https://doi.org/10.1007/s11243-026-00714-5","journal":{"identity":"transition-metal-chemistry","isVorOnly":false,"title":"Transition Metal Chemistry"},"publishedOn":"2026-02-18 15:57:14","publishedOnDateReadable":"February 18th, 2026"},"versionCreatedAt":"2025-10-24 23:56:39","video":"","vorDoi":"10.1007/s11243-026-00714-5","vorDoiUrl":"https://doi.org/10.1007/s11243-026-00714-5","workflowStages":[]},"version":"v1","identity":"rs-7831507","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7831507","identity":"rs-7831507","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}