Spent coffee ground–calcium alginate biosorbent for adsorptive removal of methylene blue from aqueous solutions

Organic dyes dissolved in water are of major concern, due to their characteristic persistence and accumulation in the environment and living organisms, leading to harmful effects such as mutagenicity, and carcinogenicity. To address this concern, this study aimed to explore the effectiveness of a bio-adsorbent prepared from spent coffee grounds and calcium alginate, referred to as SCG_ALG, in the removal of methylene blue (MB) from water. SCG_ALG exhibited an impressive Langmuir maximum adsorption capacity of 1601.85 mg g−1, besides a strong stability within a pH range of 2–10 and outstanding cyclability. The adsorption process was examined through kinetics and adsorption data, which were best fitted to PFO and Temkin's models, indicating weak physicochemical interactions. The thermodynamic study confirmed that the adsorption was a physisorption process. The possible interaction mechanism between MB and SCG_ALG was proposed through XPS spectroscopy and pH analysis. Electrostatic and π–π interactions were suggested to be involved in the adsorption mechanism. As a result, SCG_ALG emerges as a promising, cost-effective, environmentally friendly, and non-toxic adsorbent for removing MB from water-sourced and natural sources, highlighting the easy separation of the material from the solution, owing to its high adsorption capacity, reusability, and broad applicability.