Adsorption of Bisphenol-S from water using natural sorbents

Bisphenol-S (BPS), is an analog of bisphenol-A, and a serious endocrine-disrupting chemical. It is used in the production of epoxy resins, in canned foods, and baby bottles. BPS was detected in human urine samples from seven countries, with the U.S. samples having the highest level of 0.299 ng BPS/mL urine. Land-applied wastewater biosolids are bisphenol sources. Thus, this research investigates BPS adsorption onto natural adsorbents: clay minerals and biochar. Conducted in organic-free water, batch sorption studies investigated the sorption of 10 ppm BPS onto the sterilized kaolinite and montmorillonite clay minerals at 1:4, 1:5, 1:0 and 1:12 clay (g): BPS (mL) (solid-to-solution) ratios, and biochar at 5 mg/L, 10 mg/L, 12.5 mg/L, 20 mg/L, and 50 mg/L slurry concentrations. The maximum BPS removals for kaolinite and montmorillonite were 8.5% (0.055 mg BPS/g kaolinite at 1:5 clay: BPS ratio) and 48% (0.192 mg BPS/g montmorillonite for 1:4 clay: BPS ratio) respectively. Kaolinite demonstrated minimal BPS sorption at other clay doses because of strong interlayer hydrogen bonding, clay particle aggregation. Increases in the montmorillonite clay dose resulted in an increase in the BPS removed due to an availability of sufficient BPS adsorption sites. At 0.1 M and 0.3 M ionic strengths- CaCl2, KCl, NaCl, montmorillonite clay showed zero BPS adsorption at 1:2 and 1:10 clay: BPS ratios. The electrolyte\u27s cations blocked the adsorbent\u27s surface pores, destabilized and aggregated the clay particles, so BPS could not access the adsorbent\u27s interlayer sorption sites. Kinetics results (1-day, 3-day, and 5-day tests with 0.1 M NaCl) indicated that the adsorbent was saturated due to water molecular intercalation (strong hydrogen bonding) at high and low clay: BPS ratios. BPS adsorption onto montmorillonite (1:2 and 1:10 clay: BPS ratios; 0.3 M ionic strengths CaCl2 background electrolyte) decreased as the solution pH increased from 5 to 9. At pH 5, the maximum BPS removal for 1:10 ratio was 8.4% (0.083 mg of BPS/g montmorillonite), and 5.9% for 1:2 ratio (0.012 mg of BPS/g montmorillonite). At the 1:2 ratio (higher clay dose), adsorbent agglomeration occurred, and limited BPS sorption. Further increase in solution pH increased the magnitude of the adsorbent\u27s negative surface charge (pH \u3e pHpzc), causing stronger attraction of electrolyte cations. At pH 9, repulsive forces between montmorillonite and BPS (pH \u3e pKa) favored no BPS adsorption. Overall clay mineral batch sorption studies indicated montmorillonite as a better adsorbent than kaolinite due to montmorillonite\u27s higher surface area (3.81 m2/g), greater cation exchange capacity (76.4 meq/100 g), and better shrink/swell property. BPS adsorption onto clay minerals was dominated by the electrostatic interactions between the adsorbent and the electrolyte in the solution. Batch adsorption studies on wood-based biochar (0.11 ash fraction) yielded no BPS adsorption. The biochar\u27s surface oxygen functional groups diminished the p-p interactions between the biochar and BPS, and decreased the biochar\u27s hydrophobicity, which favored strong hydrogen bonding with the water molecules. Therefore, preliminary tests on biochar\u27s ash content and surface functional groups need to be investigated before its use as a soil amendment to remove target organic contaminants