Utilizing eco-friendly kaolinite-biochar composite adsorbent for removal of ivermectin in aqueous media

Several emerging contaminants are currently used in an unregulated manner worldwide, resulting in their increasing stringent limits in water by regulatory bodies. Thus, more viable and cheap treatment technologies are required. Recently, synergistic combinations of low-cost adsorbents have shown huge potential for aqueous toxic metals adsorption in water treatment processes. However, there is dearth of data on their potential for emerging contaminant removal. Here, low-cost kaolinite (KAC) clay was synergistically combined with blended Carica papaya or pine cone seeds, and calcined to obtain composites of KAC-Carica papaya seeds (KPA) and KAC-pine cone seeds (KPC). These adsorbents were characterized and evaluated for ivermectin adsorption at varying operating times (15-1440 min), pH (3-11), concentration (100-600 μg/L), and temperature (19.5-39.5 °C), as well as testing adsorbents' reusability. The composites exhibited marked property differences including over 250% cation exchange capacity increases and ≥50% surface area decreases, but unchanged KAC clay primary lattice structure. Ivermectin adsorption data were explained using kinetics and adsorption isotherm models. The rate of adsorption on KAC decreased over time, while rates for KPA and KPC increased until equilibrium at 180 min; the presence of biomaterials in the composites conferred better ivermectin adsorption and retention under continuous agitation. The adsorbents exhibited dual adsorption peaks one each at the acidic and alkaline pH regions as solution pH changed from 3 to 11. The rate data fitted (≥0.9232) the homogeneous fractal Pseudo-Second Order (FPSO) better than any other kinetics model, as well as the Freundlich adsorption isotherm model (≥0.9887); these indicate complex interactions between ivermectin and the adsorption sites of both composites. Ambient temperature increase up to ≈30 °C caused higher ivermectin adsorption but beyond this temperature there was drastic drop in adsorption. The KPA and KPC adsorption capacities are 105.3 and 115.8 μg/g, respectively. The KPC was better at reducing ivermecitn in low-concentration solution (≈75 μg/L) to less than 5.0 μg/L compared with KPA with ≈20.0 μg/L. Though KPC showed better efficiency in adsorption capacity and lowering concentration in low-concentration solutions, KPA exhibited better reusability with 83.5 and 67.5% initial adsorption strengths remaining in the second and third adsorption cycles, respectively, compared to the 73.8 and 58.8% for the KPC. These results indicate that KPA and KPC composites have the economic potential for application in water treatment processes.