Sorption of micropollutants to hydroponic substrata: Effects of physico-chemical properties

Constructed wetlands could be improved with novel substrata to enhance the removal efficiency of micropollutants from wastewater and reduce the spatial footprint. Use of hydroponic substrata like mineral wool, pumice, wood - and coconut fibre is both novel for wastewater treatment, and potentially suitable, due to their hydraulic compatible texture. Since sorption is a key removal process for pollutants in constructed wetlands, this study investigates sorption to novel substrata by six priority micropollutants, caffeine, carbamazepine, ibuprofen, naproxen, sulfamethoxazole and trimethoprim, with diverse Log K_ow and pKa values. We found that higher sorption was positively related to substrata's organic matter content. Organic substrata, namely wood - and coconut fibre, demonstrated higher sorption affinity compared with the inorganic substrata mineral wool and pumice, notably regardless of the micropollutants’ Log K_ow. Carbamazepine efficiently and rapidly sorbed onto organic substrata, which is particularly noteworthy considering the recalcitrance of carbamazepine in conventional treatment plants. The sorption isotherms were best described by the Freundlich model rather than Langmuir or linear models. We determined that using wood fibre in constructed wetlands would require around 35–66% less area to remove the selected micropollutants, compared to soil as matrix. Thus our results show the potential of using hydroponic substrata such as wood - and coconut fibre to both remove micropollutants in, and reduce the footprint of constructed wetlands.