Removal of microcystin-LR and other water pollutants using sand coated with bio-optimized carbon submicron particles: Graphene oxide and reduced graphene oxide

Abstract

Sand is the most used filter adsorbent material in a drinking water treatment plant (DWTP) or household purpose filters. However, sand poses challenge for the micropollutant removal in presence of metals, natural organic matter, total organic carbon, ammonia and other macro-pollutants. In this study, sand was coated using laboratory-synthesized graphene oxide (GO) and reduced graphene oxide (rGO) to enhance its surface properties in terms of hydrophobicity, roughness and specific surface area (6 times higher for GO/rGO as compared to 5.5 m2/g for sand), that allowed an enhanced adsorption of macro pollutants as well as one target micropollutant: Microcystin-LR (MC-LR). A more electropositive surface than sand (zeta potential: −43.2 mV) in form of iron oxide-coated sand (IOCS or Fe: −21.2 mV) and its combination with GO (FeGO: −13.4 mV) was tested to validate the hypothesis of enhanced MC-LR adsorption due to electrostatic attraction. Two known MC-LR degraders (Arthrobacter ramosus and Bacillus sp.) were screened before bioaugmentation, based on the biofilm forming potential for each coated sand composite. Additionally, the dosage of GO and rGO were bio-optimized before coating them over the sand grains (400 mg/L for GO and 520 mg/L for rGO), to obtain a non-toxic and non-disruptive effect and providing at least 60% cell viability. A highest MC-LR removal of 91% was obtained under biodegradation phase using rGO-coated sand that showed an increase of 47.2% in MC-LR removal when compared to physical adsorption phase. Sand filter (control) showed a maximum MC-LR removal of 54.7%. The highest saturation adsorbent constants of 8.5 mg/kg and 7.4 mg/kg were obtained for GO and rGO-coated sand media, respectively, which was 5–6 times higher than the uncoated sand.