From Waste to Resource: Surface-Engineered Spent Coffee Grounds as a Sustainable Adsorbent for Oil–Water Separation

Oil pollution in water is a serious issue worldwide that urgently requires an efficient and cost-effective method for separating oil–water mixtures. Selective adsorption using solid adsorbents is a promising approach. However, conventional oil-cleaning adsorbents, which are typically derived from non-renewable resources, are expensive, environmentally harmful, incapable of separating oil–water emulsions, or inefficient in recovering the adsorbed oil. Thus, we developed a sustainable method for cleaning oil pollution that addresses environmental challenges and utilizes functionalized organic waste. Superhydrophobic SiO2 nanoparticle-fabricated spent coffee grounds (SSD-SCGs) were developed, which rapidly adsorb various oils in water, including viscous crude oil, in both ambient and harsh chemical environments, with 98.5–99.7% removal efficiency. Our separation method is an energy-efficient, single-unit, flow-through, and gravity-driven system. Furthermore, the unique wettability and hierarchical porosity of the SSD-SCGs enable the effective separation of oil–seawater emulsions. The recovery of adsorbed oils and regeneration of the adsorbent were easily achieved through mechanical compression. After multiple uses, the SSD-SCGs were converted into coffee logs, a fuel source with a high heat value, thereby realizing the goal of converting waste to energy. A life-cycle assessment revealed that the SSD-SCGs had greater positive environmental and economic impacts than conventional SCGs. This study reveals an avenue for converting agricultural waste into advanced functional materials to address multiple challenges related to the United Nations Sustainable Development Goals.