Valorization of agro-waste biomass as composite adsorbents for sustainable wastewater treatment

Agro-waste biomass in form of torrefied wheat straw (S) and oat hulls (Oh) were combined with kaolinite (K) and chitosan (Ch) to obtain pelletized adsorbent materials. The agro-waste pellets were compared to a common food-waste material in form of spent coffee grounds (SCG) as a reference biomass system. The composites were prepared with variable biomass content via physical blending and compared against their epichlorohydrin (ECH) crosslinked counterparts. IR and 13C NMR spectroscopy were employed for structural characterization of the composites, along with complementary thermogravimetric analysis. Composites with 80% Oh content were unstable and required ECH crosslinking, whereas their SCG and S counterparts were stable without crosslinking. A cationic methylene blue (MB) dye probe was employed to evaluate the adsorption properties of the pelletized materials. The Sips model was used to estimate the adsorption capacity (Qm), which was correlated with increasing biomass content: Qm = 22 ̶ 34 mg/g for the Oh composites, Qm = 21 ̶ 86 mg/g for the S composites, and Qm = 18 ̶ 98 mg/g for the SCG composites. Crosslinking notably reduced the adsorption capacity for both Oh and S composites, whereas greater MB uptake occurred for the SCG composites (Qm ∼ 135 mg/g). A cost-benefit analysis showed that S composites are competitive adsorbents for MB per dollar material cost versus SCG composites. Composites that contain torrefied biomass have lower chemical input costs that afford the design of sustainable adsorbent technology in a pelletized form.