• Shifting sands: the pH-dependent removal of emerging contaminants using sustainable materials

Concerns over contaminants of emerging concern (CECs) in wastewater effluent have renewed interest in low-energy sustainable treatment solutions. The authors evaluated the removal of two widely co-detected antibiotics, sulfamethoxazole (SMX) and trimethoprim (TMP), in two bench-scale columns—one packed with sand alone and one supplemented with modified sawdust adsorbent. Nine influent conditions were tested, single CECs at 1 mg·L⁻¹, both CECs in a mixture (0.5 mg·L⁻¹ each), and single CECs at 0.5 mg·L⁻¹, across three different pH levels. Samples were collected (n=3) and concentrations were quantified by high-performance liquid chromatography. The sand-only columns exhibited high TMP removal at both influent dosages (-81.4%) when alone but decreased markedly to -42.2% ± 3.3 in the presence of SMX, indicating competitive or antagonistic effects. However, the addition of treated sawdust doubled this TMP removal from antibiotic mixtures to -85.3%. SMX-alone demonstrated leaching in sand (+19.3% ± 31.3), and sawdust amendment did not improve its removal, increasing concentrations 70.3% ± 106.4 in the TMP+SMX mixture. When the pH was modulated, TMP removal decreased with increasing influent acidity, with statistically significant differences observed (Welch’s t-test, p < 0.05), confirming pH-dependent interactions. In contrast, SMX removal improved under acidic conditions, though removal at pH 3 did not differ significantly from that at pH 4 in either single- or dual-media filters. Fourier-transform infrared (FTIR) analysis of adsorbent materials demonstrated functional groups, and it subsequently explained the interactions controlling adsorption; hydrophobic, electrostatic, and hydrogen bonding interactions were possible due to non-polar, carboxyl, and carboxylic acid groups present, respectively, in the silica (sand) and lignocellulosic (sawdust) materials. Our initial results indicate that lignocellulosic amendments can enhance slow-sand filter removal of resistant compounds such as SMX. Ongoing pilot-scale experiments assessing biofilm development and long-term performance will probe mechanisms and scale-up feasibility.