Sorption and desorption of diverse contaminants of varying polarity in wastewater sludge with and without alum

Abstract

Sewage sludge sorption and desorption measurements were conducted for nine diverse contaminants of varying polarity: caffeine, sulfamethoxazole, carbamazepine, atrazine, estradiol, ethinylestradiol, diclofenac, and, for the first time desethylatrazine and norethindrone. Two types of sorption behaviour were observed. Compounds with a log octanol–water partition coefficient, log K_ow, below 3 showed little or no sorption over 48 hours of shaking, while compounds with log K_ow over 3 showed 30 to 90% sorption within the first few minutes. After 6 hours of shaking, mass loss through suspected biotransformation became evident for some compounds. At the pH range considered (5.7–6.7), diclofenac (pK_a 4.0, log K_ow 4.5) was the only compound in which pH dependent sorption could be quantified. The log sewage sludge-water distribution coefficients, log K_d, ranged from 0.2 to 2.9, and, as expected, increased with increasing log K_ow of the compound and organic carbon (OC) content of the sewage sludge. A sewage sludge precipitated with alum had a substantially lower K_d values, as well as lower OC content, compared to alum-free sludge. Desorption was studied by sequentially replacing supernatant water. With each water replacement, log K_d values tended to either remain similar (following a linear isotherm) or in some cases increase (following a Freundlich-type isotherm). The length of time required to restore equilibrium increased with each rinsing step. A literature review of reported K_d values compared well with the alum-free sludge data, but not the alum-sludge data. Sewage sludge K_d across the literature appear more consistent with increasing K_ow.