Algal biochar for metalloid removal: from waste algae to cleaner water

Abstract

Arsenic (As), antimony (Sb), selenium (Se), and tellurium (Te) are released widely by mining, smelting, fuel combustion, and agrochemical use. These metalloids persist, bioaccumulate, and remain toxic at trace concentrations. Affordable and sustainable remediation options are therefore needed. Algal biochar, produced from microalgal and macroalgal biomass through thermochemical conversion, has emerged as a candidate adsorbent. Its appeal rests on a mineral-rich ash fraction, abundant surface functional groups, low feedstock cost, and compatibility with circular-bioeconomy principles. This review provides a comprehensive synthesis of recent advances in algal biochar production, physicochemical characteristics, surface modification strategies, and mechanisms governing metalloid removal. Literature published between 2006 and mid-2026 was identified, screened, and selected using the PRISMA 2020 reporting framework, and the evidence was synthesised narratively; no meta-analysis was performed. The synthesis examines how feedstock type, production conditions, and solution chemistry influence adsorption performance. The review discusses key removal mechanisms, including electrostatic attraction, surface complexation, ion exchange, precipitation, redox transformation, and pore filling, and compares algal biochar with conventional biochar, activated carbon, and nanomaterial-based adsorbents. Applications in wastewater, groundwater, soil, constructed wetlands, and industrial effluents are assessed alongside waste-biomass valorisation and life-cycle sustainability. The evidence is strongest for As, moderate for Se, and sparse for Sb and Te. Most reported data come from single-metalloid batch experiments in synthetic solution, whereas field-scale performance, real-matrix competition, regeneration, and the fate of spent, metalloid-laden biochar remain poorly documented. These gaps, rather than adsorption capacity alone, currently limit practical deployment.