Fates of polar organic contaminants in oxic sand columns: Impacts of sand type, hydraulic retention time and biodegradable organic carbon availability

Biotransformation in sand filters and natural subsurface barriers is a key process for eliminating organic micropollutants from drinking water. However, underlying mechanisms are complex and knowledge on influencing parameters is fragmentary. There is still a knowledge gap regarding contaminants of emerging concern, such as persistent and mobile substances, and their fate in those processes. This study investigated sorption and biotransformation of 18 polar and potentially persistent contaminants in oxic sand columns, evaluating the effect of biodegradable dissolved organic carbon (BDOC, drinking water vs. surface water), hydraulic retention time (9 h vs. 25 h) and sand type (microbial community, technical sand vs. sand from a groundwater recharge site). Substance and operation specific extent of biotransformation was observed. Overall, nine substances were biodegraded, while nine indicated persistence. Negligible sorption was observed, only the cationic 1,3-di-o-tolylguanidine was retarded by four bed volumes. The highest impact on biotransformation was observed for the parameter BDOC, followed by hydraulic retention time and finally sand type. The strongest effect was observed for 2-methyl-2-propene-1-sulfonic acid (MPSA), which was readily biodegradable in surface water and persistent under BDOC limiting conditions. Previous classifications as persistent, e.g. of 2-acrylamido-2-methylpropane sulfonate and adamantan-1-amine, were confirmed. Three substances previously categorized as persistent (acesulfame, MPSA and 4-hydroxy-1-(2-hydroxyethyl)-2,2,6,6-tetramethylpiperidine) showed biotransformation. These results emphasize the complexity of persistence assessments and the potential for some polar contaminants to be eliminated in treatment processes relying on biotransformation in the subsurface or sand filters under oxic conditions.