Bacteria-mediated green synthesis of silver and zinc-oxide nanoparticles using metal-tolerant soil isolates (Bacillus sp. PB-07 and Pseudomonas sp. ZN-12) for comparative removal of Pb(II) and Cd(II) from water

Abstract

Biogenic (bacteria-mediated) synthesis offers a sustainable route to functional nanomaterials while avoiding the harsh reductants of conventional chemical methods. Here we isolated two metal-tolerant bacteria from battery-recycling-facility soil — a lead-tolerant Bacillus sp. PB-07 and a zinc-tolerant Pseudomonas sp. ZN-12 (identified by 16S rRNA gene sequencing) — and used their cell-free supernatants to green-synthesise protein-capped silver nanoparticles (AgNPs) and wurtzite zinc-oxide nanoparticles (ZnO NPs). The two nanomaterials, together with an Ag/ZnO composite, were characterised by XRD, FTIR, FE-SEM/TEM, EDS, AFM, BET and zeta-potential analysis and evaluated comparatively for the removal of Pb(II) and Cd(II) from water. The biogenic AgNPs were face-centred-cubic Ag⁰ (surface plasmon resonance at 421 nm; crystallite size 60.1 ± 10.8 nm; BET 28.6 m² g⁻¹) carrying a negatively charged protein-carboxylate surface (ζ = −38.4 mV; pH_pzc 2.3), whereas the ZnO NPs were phase-pure hexagonal wurtzite (crystallite 15.3 ± 2.3 nm; BET 42.3 m² g⁻¹; band gap 3.24 eV; pH_pzc 9.1). Uptake by both materials was well described by pseudo-second-order kinetics (R² > 0.997). The two adsorbents differed mechanistically in a striking and internally consistent way: ZnO NPs followed the Langmuir isotherm with maximum monolayer capacities of 193.6 mg g⁻¹ (Pb) and 148.2 mg g⁻¹ (Cd) and an endothermic, entropy-gaining thermodynamic signature (ΔH° = +8.7 kJ mol⁻¹ for Pb), consistent with inner-sphere Zn–OH complexation and secondary precipitation, while AgNPs followed the Freundlich isotherm (Langmuir q_max 118.4 and 88.7 mg g⁻¹) with an exothermic, entropy-losing signature (ΔH° = −9.3 kJ mol⁻¹ for Pb), consistent with electrostatic attraction and carboxylate coordination on an energetically heterogeneous surface. ZnO NPs combined the higher capacity, a practical circum-neutral working pH, and the best regenerability (<7% capacity loss over five cycles), whereas the antibacterial biogenic AgNPs are attractive for trace-level polishing and dual-function (antimicrobial + adsorptive) treatment. The results establish bacterially-synthesised AgNPs and ZnO NPs as a tunable, sustainable adsorbent platform for heavy-metal-contaminated water.