Binary and complex metal based nanoadsorbents for water decontamination: Mechanisms, materials, and future perspectives

Water pollution due to dyes and heavy metals (HMs) demonstrate a global key challenge in the 21st century. Adsorption is considered one of the most competent techniques and metal/metal oxide nanoadsorbents have caught worldwide attention exhibiting several benefits including abundant active binding sites, tunable chemistry, functionalization, excellent regenerable capacity, and economic viability. Monometallic nanoadsorbents generally exhibit adsorption capacities between 10 mg g⁻¹ to around 200 mg g⁻¹ with slow kinetic rate, whereas, in comparison, bimetallic/complex nanoadsorbents depict significantly higher capacities (25 mg g⁻¹ to around 2000 mg g⁻¹) for the removal of dyes/HMs with rapid kinetics and > 90 % reusability over multiple cycles, depending on type of dye/metal ions, type of adsorbent and adsorption conditions. Overall, remarkable performance of binary/complex metals is evident owing to synergistic interactions between different metals of an adsorbent. Additionally, bibliometric analysis depicts a worldwide increase in interest for the exploration of bimetallic/complex nanoadsorbents, specifically after 2018. This review also facilitates deep insights into synthesis approaches, adsorption mechanisms and performances of these nanoadsorbents for the removal of dyes/HMs and proposes an eco-safe engineering roadmap for real-world deployment. The major research gaps identified from the consolidated research data involve lack of investigations under real water systems, absence of standardized characterization framework for correlation between structures and adsorption performances, and inadequate evaluation of leaching concerns, environmental impacts, and end -of-use management. Bridging these gaps through advanced modelling approaches and environmental assessment frameworks is very necessary to transform laboratory scale success into sustainable, real life water treatment applications.