Utilizing low-cost natural waste for the removal of pharmaceuticals from water: Mechanisms, isotherms and kinetics at low concentrations

The use of abundant natural wastes as environmentally friendly products promotes a circular green economy and cleaner production. The potential use of natural waste materials without additional processing for the removal of priority pharmaceuticals from water was investigated. Here, the performance of selected low-cost biosorbents (biochar, macro-algae and wood chippings)was evaluated using two extensively prescribed model pharmaceuticals: diclofenac (DCF)and trimethoprim (TMP). The physicochemical properties of the biosorbents were examined (to shed light on likely biosorption mechanisms)using Brunauer–Emmett–Teller (BET)measurements, scanning electron microscopy (SEM), zero point of charge (pH _zpc )measurements and Fourier transform infrared spectroscopy (FTIR). Experimental data from kinetic studies fitted a pseudo-second order model, and multiple diffusion steps limited the mass transfer of analytes. Intra-partile diffusion was the rate limiting step for biochar, while macro-algae and wood chippings were limited (mainly)by adsorptive attachment. The equilibrium data for most of the studied systems best fitted a Langmuir model, while the Freundlich model provided a better fit for TMP with wood chippings. At μg∙L ⁻¹ initial pharmaceutical loading levels, the maximum biosorption capacity for DCF was attained with biochar (7.25 × 10 ³ μg g ⁻¹ ), while macro-algae performed best for TMP (7.14 × 10 ⁴ μg g ⁻¹ ). Both chemical and physical interactions were likely responsible for the biosorption of pharmaceuticals. High removal efficiencies were achieved at the low initial loadings studied, indicating the potential application of those sustainable low-cost biosorbents at low (environmentally relevant)pharmaceutical concentrations.