Presence of Humic Acids in Landfill Leachate and Treatment by Flocculation at Low pH to Reduce High Pollution of this Liquid

Humic substances (HSs) are abundant in landfill leachate, and specially humic acids (HAs), which are insoluble at low pH in aqueous solutions. Attending to chemical properties of humic acids, we describe in this work a new method for a sustainable treatment of landfill leachate originated from solid wastes which consists in the reduction of organic load (COD) and colour and is based in the gradual decrease of pH to the value in which HAs are insoluble in water solution. Zeta Potential value marks the chemical stage of humic acids, because of ionization-protonation of the phenolic and carboxylic groups and this parameter is monitored during flocculation, changing from -16.8 mV at pH 7.7 to 0.0068 mV at pH 2.0, when HAs precipitate. The final result is the reduction of the organic matter content (COD) and colour in the leachate in 86.1 % and 84.7 %, respectively. Solids produced by precipitation during the acidification treatment have been characterized by CHNS analysis and FT-IR, concluding a high similarity in chemical composition with commercial and natural HAs. Protonated HAs at low pH can interact with other molecules by hydrogen bonds and form bigger molecular structures much more unstable in suspension which conduct to precipitation. Mean diameter of the HA aggregates was measured, detecting the formation of big molecular structures at low pH.