Natural biosorbents for the removal of metals from aqueous solutions

Abstract

The removal of metals from potable and wastewater to regulatory standards presents unique challenges. Conventional treatments, chemical precipitation, adsorption by activated carbon, ion-exchange, electro-chemical and reverse osmosis can be expensive or ineffective at low concentrations. Recent research has shown that biosorption by low cost biomass can be an effective alternative. Biosorption is the passive (non-metabolic) uptake and concentration of metals by non-viable biological materials. In this work, natural materials and shellfish processing wastes were physiochemically characterised to identify products with suitable characteristics to perform as biosorbents. Brown seaweed (FllCliS vesiculosus), Douglas fir wood bark (Pselldolsllga menziesil), peat and carapace from the edible crab (Cancer pagurus) were assess~d for Cu(II) removal as a function of time and concentration. Mechanically and commercially milled carapace (MMC and CMC) were the most efficient, removing >95.0 % within 40 minutes from 100-1000 mgIL Cu(II) solutions. Extended studies showed MMC was also applicable for use in biosorption columns and effective for the removal of Cd(II), Hg(II), Pb(II), Ni(lI) and Zn(II), at concentrations ranging from 1-1000 mgIL. MMC and CMC were compared with chitin, chitosan, Darco© and Norit© activated carbons and Dowex© ion-exchange resin for the removal of Cu(II). Dowex© and Norit© were the most efficient commercial sorbents removing up to 99.9 % from the 1-1000 mgIL. MMC and CMC compared favourably and were effective over the 1-4000 mg/L range and suitable for use in acidic solutions (pH 4.2-4.7). Sorption of Cu(II), Hg(II), Pb(II) and Zn(II) by MMC were predicted reliably (R2>0.99) using a pseudo-second-order model, with rates of 1.34, 14.6, 1.37 and 1.30 mg/mg/min respectively. An intra-particle diffusion. model and SEM-EDAX microanalysis revealed that the biosorption of metals proceeds via rapid adsorption-precipitation to surface binding sites, followed by rate limiting intra-particle diffusion. Equilibrium uptakes were evaluated using the Langmuir, Freundlich and Redlich-Peterson adsorption isotherm models. The best fits were obtained for the Langmuir model with 416.7, 86.2, 30.5 and 14.6 mg/g MMC for Pb(II), Cu(II), Zn(II) and Hg(II) respectively. The results show that biosorption of metals is complex and proceeds via adsorption, precipitation, absorption, and ion exchange depending on pH, initial concentration and material conditioning. Although CMC was marginally more efficient than MMC, MMC is less labour intensive and more cost effective to produce. Therefore, MMC offers significant potential as a viable metal biosorbent and merits further development.