Heavy Metal Contamination in Soil, Water, and Biota: Sources, Impacts, Remediation Strategies, and the Emerging Role of Trifolium spp. with Valorized Eggshell and Bagasse Biosorbents

Heavy metal contamination of soils, water bodies, and food crops represents one of the most persistent and escalating environmental challenges of the twenty-first century. Driven by rapid industrialization, intensive agriculture, urbanization, and mining, metals including lead (Pb), arsenic (As), cadmium (Cd), mercury (Hg), chromium (Cr), nickel (Ni), copper (Cu), and zinc (Zn) have accumulated in ecosystems worldwide, posing severe risks to soil integrity, plant productivity, and human health through bioaccumulation and biomagnification across food chains. This review synthesizes global trends in heavy metal contamination from 1990 to 2025, traces the evolution of remediation strategies from conventional physicochemical treatments toward integrated green and hybrid approaches, and evaluates the role of Trifolium spp. (clovers) as phytoremediators. Special attention is given to Trifolium repens (white clover) and its documented capacities for metal uptake, rhizosphere enhancement, and nitrogen fixation. The review further examines the emerging waste-to-resource paradigm employing valorized chicken eggshell and sugarcane bagasse as low-cost biosorbents, including their individual and synergistic performance in removing Pb, Cd, As, and Hg from contaminated matrices. Regional findings from Durban, South Africa, illustrate how WWTP effluents and riverine contamination translate into tangible soil and crop exposure risks, underscoring the urgent need for locally appropriate, sustainable remediation solutions. The integration of Trifolium spp. with eggshell- and bagasse-derived amendments is proposed as a promising low-cost, green technology framework for contaminated land management in resource-constrained settings.