Occurrence and transformation of phosphonates in textile dyeing wastewater along full-scale combined treatment processes

Phosphonates discharged from wastewater treatment plants (WWTPs) have attracted increasing concerns because of their potential impact on eutrophication and potential risks to aquatic ecosystems. However, very few studies are available on their occurrence and transformation in WWTPs, partly due to the lack of sensitive methods for phosphonate analysis in complex matrices. Herein, based on our recent progress in phosphonate analysis, the occurrence and transformation of phosphonates along the full-scale wastewater treatment processes of two textile dyeing WWTPs were revealed. A set of typical phosphonates, including six phosphonate chelators (PCs) and four potential degradation products of PCs (DP-PCs) were quantified in different units and the final dewatered sludge. Three PCs (2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC), 1-hydroxyethane 1,1-diphosphonic acid (HEDP) and nitrilotris(methylene phosphonic acid) (NTMP)) at upmost mg/L and a considerable amount of four DP-PCs (9.12-608 μg/L) were detected in the influents of both WWTPs. In the subsequent treatment, NTMP could be removed more efficiently than PBTC and HEDP, especially in the coagulation unit, and the dissolved phosphonates were eliminated more readily than other dissolved organic phosphorus fractions. Of particular note, the toxicologically critical DP-PC (i.e., aminomethylphosphonic acid) was produced during the coagulation and biological treatment units. The final precipitation unit seemed essential to ensure satisfactory removal of PCs and DP-PCs. In addition, a significant accumulation of phosphonates in dewatered sludge (up to 7.81 g/kg) and the widespread occurrence of harmful DP-PCs also reminded us to pay more concerns on their potential risks during further sludge disposal in future.