A Review of E-Waste Treatment Technologies from the Perspective of Heavy Metal Release: Efficiency, Mechanisms, and Environmental Risks from Informal Acid Leaching to Formal Smelting

The global proliferation of electronic waste (e-waste) has raised critical environmental and public health concerns, primarily stemming from the liberation of toxic heavy metals during recycling operations. This comprehensive review provides a systematic comparative analysis of two dominant e-waste treatment pathways—informal acid leaching and formal smelting—evaluating their metal release efficiencies, underlying mechanisms, and associated ecological consequences. Informal acid leaching, widely practiced in developing regions through the application of concentrated acids, facilitates metal extraction but leads to extensive soil and aqueous contamination, owing to unregulated effluent discharge and absent mitigation measures. Formal pyrometallurgical smelting, conversely, employs regulated thermal treatment in enclosed environments, augmented by sophisticated gas cleaning and residue management systems, achieving high metal recovery rates while substantially curtailing environmental emissions. Nevertheless, widespread implementation of formal techniques is impeded by operational heterogeneities, divergent pollution monitoring methodologies, and contextual socioeconomic constraints. The study underscores the necessity of developing harmonized assessment protocols, blended policy instruments, and flexibly scalable technological interventions to alleviate heavy metal pollution and advance sustainable e-waste management strategies worldwide.