Plant Extracts for Heavy Metal Removal from Urban Wastewater: A Critical Review of Removal Mechanisms, Toxicity Reduction, and Implementation Constraints

Abstract

Urban wastewater carries a persistent load of heavy metals from households, commercial activity, road runoff and small industries. Conventional treatment moves much of this load into sewage sludge, but a substantial fraction can remain in the treated effluent, which limits safe water reuse and the agricultural use of biosolids. Extracts obtained from plants, including seed proteins, cactus and okra mucilages, condensed and hydrolysable tannins, saponins and polyphenol-rich leaf extracts, have been proposed as low-cost and biodegradable alternatives or supplements to metal salts, synthetic polymers and commercial sorbents. This critical narrative review examines whether that proposal is supported for metal removal from urban wastewater specifically, rather than for turbid surface water or concentrated synthetic solutions. Literature published between 2000 and mid-2026 was identified through multidisciplinary, biomedical and regional scholarly indexes, supplemented by citation tracking, and was appraised for matrix realism, analytical adequacy, mechanistic support and reporting quality. The evidence shows that plant extracts act through several distinct routes: charge neutralisation and bridging of metal-bearing particles, complexation by carboxyl, hydroxyl, amine and catechol groups, ion exchange, and reduction of hexavalent chromium by polyphenols. Performance is reproducible for particulate and iron-associated metals and for chromium removal by tannin-derived materials under acidic conditions. It is considerably weaker and less consistent for dissolved cadmium, nickel, zinc and lead at the low concentrations typical of municipal effluent. Most studies used spiked single-metal solutions, reported removal percentages without mass balances, and fitted kinetic and isotherm models in ways that do not establish mechanism. Seed-based coagulants also release dissolved organic matter, and many so-called green nanomaterials owe their activity mainly to iron rather than to the plant extract. The strongest case at present is for plant extracts as coagulant aids that reduce inorganic coagulant demand, as tannin-based coagulants at pilot scale, and as reductants or precursors for chromium-selective materials. Establishing a role for plant extracts in urban wastewater management will require standardised extract characterisation, tests in real effluents at environmentally relevant concentrations, organic-carbon and toxicity accounting, and continuous-flow evaluation.