Comprehensive evaluation of a slag–activated carbon composite for metformin removal from wastewater and its agricultural reuse potential

Abstract

Metformin, a widely prescribed antidiabetic drug, persists in aquatic ecosystems and poses toxic risks to nontarget organisms. This study synthesized a novel composite adsorbent from steel slag and activated carbon (2 : 1 ratio) for efficient metformin removal from water, then evaluated the spent adsorbent as a soil amendment. The composite was characterized by XRD, FTIR, BET, SEM-EDX, and particle size analysis. Batch adsorption experiments examined pH (3–9), adsorbent dose (0.025–0.3 g), contact time, initial concentration, and temperature (25–55 °C). Kinetic data were fitted to pseudo-first-order, pseudo-second-order, mixed 1,2-order, Avrami, and intraparticle diffusion models; equilibrium data followed Langmuir, Freundlich, Sips, Dubinin–Radushkevich, Redlich–Peterson, and Baudu isotherms. Thermodynamics indicated spontaneous, endothermic adsorption with a maximum capacity of 112.5 mg g−1. In agronomic trials, the unspent SAC composite (T5) delivered the best performance across all parameters. Conversely, the metformin-loaded spent composite (T6) caused marked phytotoxicity: plant growth was not measurable at initial sampling, appearing later and was severely delayed and weaker than even the unamended control. Despite these growth effects, both T5 and T6 substantially improved soil quality. Compared with the control (CEC 3.1 cmolc kg−1), T5 and T6 increased cation exchange capacity by 294% (12.2) and 313% (12.8 cmolc kg−1), respectively. T5 raised soil pH from 6.9 to 7.6, enhanced macronutrient availability, and reduced heavy metal solubility. Soil organic matter increased six-fold (0.4% to 2.4%). Available phosphorus rose by 133% (4.2 to 9.8 mg kg−1) and available potassium by 88% (32.7 to 61.4 mg kg−1) in T5, attributed to iron-phosphate release and enhanced cation retention. The composite also immobilized heavy metals effectively. Overall, while the spent composite offers strong soil conditioning benefits, its agricultural use is critically constrained by metformin phytotoxicity, requiring application rate optimization.