Designer biochar with enhanced functionality for efficient removal of radioactive cesium and strontium from water

Radioactive elements released into the environment by accidental discharge constitute serious health hazards to humans and other organisms. In this study, three gasified biochars prepared from feedstock mixtures of wood, chicken manure, and food waste, and a KOH-activated biochar (40% food waste + 60% wood biochar (WFWK)) were used to remove cesium (Cs⁺) and strontium (Sr²⁺) ions from water. The physicochemical properties of the biochars before and after adsorbing Cs⁺ and Sr²⁺ were determined using X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopy, extended X-Ray absorption fine structure (EXAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDX). The WFWK exhibited the highest adsorption capacity for Cs⁺ (62.7 mg/g) and Sr²⁺ (43.0 mg/g) among the biochars tested herein. The removal of radioactive ¹³⁷Cs and ⁹⁰Sr exceeded 80% and 47%, respectively, in the presence of competing ions like Na⁺ and Ca²⁺. The functional groups present in biochar, including -OH, -NH₂, and -COOH, facilitated the adsorption of Cs⁺ and Sr²⁺. The Cs K-edge EXAFS spectra revealed that a single coordination shell was assigned to the Cs-O bonding at 3.11 Å, corresponding to an outer-sphere complex formed between Cs and the biochar. The designer biochar WFWK may be used as an effective adsorbent to treat radioactive ¹³⁷Cs- and ⁹⁰Sr-contaminated water generated during the operation of nuclear power plants and/or unintentional release, owing to the enrichment effect of the functional groups in biochar via alkaline activation.