Dual role of biochar for heavy metal removal from municipal solid waste incineration fly ash wash water and its reuse in cement mortar

초록

Water washing is as an effective pre-treatment method for municipal solid waste incineration fly ash (MSWIFA) to remove chlorides; however, it generates a secondary waste stream in the form of heavy-metal-laden wash water. This study proposes a dual-function utilization strategy for biochar, wherein biochar is first used to adsorb heavy metals from MSWIFA wash water and subsequently reused as a partial cement replacement in mortar. Simulated fly ash washing water was prepared by mixing MSWIFA with water at a 1:20 wt ratio and thereafter treated with high-temperature woodchip-derived biochar. Heavy metal removal and adsorption-induced changes in biochar were assessed through inductively coupled plasma analysis, together with physicochemical and microstructural characterization techniques. The resulting metal-laden biochar was incorporated into cement mortar at 1% cement replacement, and its impact on fresh properties, mechanical performance, microstructural characterization and heavy metal leaching were systematically investigated. Biochar effectively reduced Pb concentration by 98.9% and completely removed Zn, while Cr removal reached 49.3%. Surface characterization demonstrated significant 31.8% reduction in BET surface area after adsorption. Biochar incorporation decreased mortar flowability by 5-8% and delayed the initial setting time by around 3%, without affecting final setting time. Compressive strength enhanced at all curing ages, with metal-laden biochar improving 3 days strength by 18.91% and untreated biochar enhancing 28 days strength by 13.89%. Toxicity characteristic leaching tests confirmed effective immobilization of heavy metals within the cement matrix. Overall, this study demonstrates a sustainable closed-loop approach for treating MSWIFA wash water and safely reusing spent biochar in cementitious composites.

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