Polluted water contaminated by heavy metals has received increasing attention due to their high toxicity, persistence in sediment, and biological accumulation of the heavy metals, which lead to a potential threat to animals and human beings even at low concentration. Traditional techniques to remove heavy metal are high cost or making the second pollution that limited large-area applications. Most importantly, traditional techniques cannot apply to such low concentration in water. Therefore, it is emergency to find a low-cost, efficiency and eco-friendly method to deal with this kind of problem. Biosorption can be a promising option in this case due to its high efficiency and eco-friendly, especially the abundant materials in the world such as biomass.
On the other hand, a huge amount of biomass waste in the world will cause an environmental problem if not handled properly. Furthermore, cellulose, hemicellulose and lignin are abundant in biomass wastes that can be employed as cheap adsorbents due to their special physical and chemical properties. The presence of well-known functional groups in these natural components turn them as potential materials for heavy metal interaction and subsequent removal.
Therefore, biomass from agricultural wastes and wood industry have been checked as natural adsorbents to solve two serious environmental problems: firstly the disposal of agriculture wastes, and secondly its use as adsorbents for the removal of heavy metals from wastewaters.
With this purpose, many different types of biomass feedstocks and some of their biochars are used to remove Cr(III), Cd(II), Cu(II) and Pb(II) ions from a mixture of multiple heavy metals. Furthermore, TiO2 nanoparticles are also a good adsorbent for removing heavy metals. Pine and pyrolyzed pine loaded with TiO2 (Pine/TiO2) have been used as sorbent for the removal of heavy metal ions from aqueous solution. Single and multi-element systems are used for the heavy metals removal. In addition, Cr(III) and Cr(VI) speciation has been checked for pine biomass/biochar systems.
In all cases, different parameters of the biosorption processe are optimized in batch systems (pH of the solution, the initial concentration and the contact time), and kinetics and isotherm modelling have been performed to elucidate the possible biosorption mechanisms. Surface morphology of the adsorbents are analyzed using scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). Moreover, X-ray absorption spectroscopy (XAS) is performed to study the biosorption mechanism at the molecular level.
The adsorption capacity of biomass is ranked as follows: FO (from industry sludge waste) > ZO (from agriculture corn biomass waste) >> CO (from wood poplar biomass waste). Complexation and cation exchange have been found to be the two main adsorption mechanisms in systems containing multiple heavy metals, with cation exchange being the most significant. As a summary of the chromium speciation study by pine biomass/biochar systems, the adsorption of Cr(III) is mainly through ion exchange with the mineral components present on the biomaterials surfaces. Pyrolysis process can increase the concentration of such minerals to increase the adsorption capacity. For Pine/TiO2, together with the ion exchange also complexation with catechol can help Cr(VI) adsorption. From XAS measurements it can be concluded that the ion exchange process with carboxylic site groups is the main biosorption step, followed by the chromium reduction.
Finally, utilization of biosorbent loaded with heavy metal as brick materials is a promising way to solve disposal problem. These biosorbents are promising materials that can be applied in large scale to deal with the polluted water in the world.