pH-dependent effects of polystyrene microplastics on ciprofloxacin toxicity and uptake by ryegrass

The ubiquitous coexistence of microplastics (MPs) and antibiotics in the aquatic environment has been demonstrated, and as such, the effects of environmental conditions (such as pH) on combined toxicity of MPs and antibiotics to biota must be determined. Here, we investigated the effects of different pH on combined toxicity of polystyrene MPs- ciprofloxacin (CIP) to ryegrass growth and uptake of CIP by ryegrass. The results revealed that, regardless of with or without MPs, the neutral and basic condition decreased CIP accumulation in ryegrass roots. However, CIP contents in shoots (including leaves) increased with increasing solution pH. Relative to pH 5.3, CIP contents in shoots increase of 24.7-fold and 29.5-fold with CIP+200 nm MPs and CIP+500 nm MPs treatment at pH 8.8. Moreover, the effects of pH on root and shoot growth inhibition more obvious at pH 7.0 and pH 8.8 than at pH 5.3. Under the neutral and basic condition (pH 7.0 and pH 8.8), the ryegrass shoot length of 500 nm MPs+CIP and 200 nm MPs+CIP treatment was only 50.3%, 69.7% and 86.8%, 81.3% of the acidic condition (pH 5.3) plants, respectively. These findings implied that the uptake and toxicity of CIP with MPs in ryegrass were influenced by solution pH. Furthermore, the relatively higher CIP removal efficiency imply the potential for CIP removal by ryegrass under MPs co-contaminated environment.