pH-Dependent Effects of Polystyrene Microplastics on Ciprofloxacin Toxicity and Uptake by Lolium perenne L.

This study investigated the impact of pH on the combined toxicity of polystyrene microplastics (MPs) and ciprofloxacin (CIP) on ryegrass growth and CIP uptake. Two different sizes of PS-COOH (200 and 500 nm) were utilized. The findings indicated that neutral and alkaline pH conditions (pH 7.0 and 8.8) significantly reduced CIP accumulation in ryegrass roots, regardless of MP presence. Conversely, CIP content in shoots (including leaves) increased with rising pH, showing 24.7-fold and 29.5-fold enhancements under CIP + 200 nm MPs and CIP + 500 nm MPs treatments at pH 8.8 compared to pH 5.3. Additionally, pH significantly influenced growth inhibition, with more pronounced effects at pH 7.0 and 8.8 than at pH 5.3. Under neutral and alkaline conditions, shoot lengths in 500 nm MPs + CIP and 200 nm MPs + CIP treatments were reduced to 50.3%, 69.7% and 86.8%, 81.3% of those at pH 5.3, respectively. These findings highlight the critical role of pH in modulating CIP uptake and toxicity in ryegrass, particularly in MP co-contaminated environments. Moreover, the relatively high CIP removal efficiency suggests the potential of ryegrass for CIP remediation in MP-polluted systems.