Remediation of heavy metals polluted soil environment: A critical review on biological approaches.

Heavy metals (HMs) pollution is a globally emerging concern. It is difficult to cost-effectively combat such HMs polluted soil environments. The efficient remediation of HMs polluted soil is crucial to protect human health and ecological security that could be carried out by several methods. Amidst, biological remediation is the most affordable and ecological. This review focused on the principles, mechanisms, performances, and influential factors in bioremediation of HMs polluted soil. In microbial remediation, microbes can alter metallic compounds in soils. They transform these compounds into their metabolism through biosorption and bioprecipitation. The secreted microbial enzymes act as transformers and assist in HMs immobilization. The synergistic microbial effect can further improve HMs removal. In bioleaching, the microbial activity can simultaneously produce H2SO4 or organic acids and leach HMs. The production of acids and the metabolism of bacteria and fungi transform metallic compounds to soluble and extractable form. The key bioleaching mechanisms are acidolysis, complexolysis, redoxolysis and bioaccumulation. In phytoremediation, hyperaccumulator plants and their rhizospheric microbes absorb HMs by roots through absorption, cation exchange, filtration, and chemical changes. Then they exert different detoxification mechanisms. The detoxified HMs are then transferred and accumulated in their harvestable tissues. Plant growth-promoting bacteria can promote phytoremediation efficiency; however, use of chelants have adverse effects. There are some other biological methods for the remediation of HMs polluted soil environment that are not extensively practiced. Finally, the findings of this review will assist the practitioners and researchers to select the appropriate bioremediation approach for a specific soil environment.

Heavy metals dispersion during thermal treatment of plastic bags and its recovery.

One of the main worries for thermal treatment of plastic bag (PB) is the air pollution resulting from heavy metal (HM) evaporation and emission. The quest of the study was to investigate their fate during thermal treatment varying with temperature and atmosphere to explore the appropriate treatment technology. Four commonly consumed polymer bags such as PE, HDPE, LDPE and PVC were selected for the analysis. The elemental compositions, heating values and total metal contents of the samples were measured by an elemental analyzer, a sulphur/halogen analyzer, a bomb calorimeter and an ICP-OES, respectively. Thermal treatments of the samples were conducted in a tube furnace at 350, 550, 650, 750, and 850 °C with 1 L/min air or N2 gas flow, respectively. 5% HNO3/10% H2O2 solution was used for absorbing metals from gas phase, and then HM distributions both in flue gas and bottom ash were determined. Results revealed that the lower heating values of HDPE, LDPE, PVC and PE bags were 33.32, 34.28, 24.82 and 36.7 MJ/kg, respectively indicating energy recovery potential. Thermal treatment showed the maximum mass reduction (>90%) of PB at 850 °C. The higher percentage of metals was distributed in ash at initial temperature that promoted to gas with rise of temperature. The used absorption solution exhibited tremendous quantity of metals recovery. However, there was no significant difference between using air and N2 gas flow during treatment of PB.

Influences of chemically controlled Ca-bearing minerals in chitosan on Pb2+ removal efficiency.

Highly purified chitosan was generally preferred for heavy metal (HM) removal and the preparation parameters varied largely without any agreement. This study investigated to the influences of chitin with different purities on the HM removal of corresponding chitosan. Sea shrimp waste was used as raw materials and Pb2+ was used as target HM. The results of orthogonal experimental analysis showed that only acid concentration played an important role in the deproteinization and demineralization processes of the chitin preparation under HCl, H2SO4 and CH3COOH treatment. Ca-bearing minerals (CBM) but not free -NH2 group of chitosan played a major role in the removal of Pb2+ from solution. Partly purified chitosan mainly removed Pb2+ by precipitation and then biosorption. The dissociation of Ca2+ from CBM elevated pH value of Pb2+ solution which benefited to precipitation and the formation of NH2-Pb2+. Partly purified chitosan prepared from HCl and CH3COOH treated chitin showed 720-753 mg/g of Pb2+ adsorption at the initial pH value of 6.0; however, highly purified chitosan prepared from HCl treated chitin showed only 45-160 mg/g. Chitosan prepared from H2SO4 treated chitin showed 720-752 mg/g of Pb2+ adsorption. This research found the unexplored information for the industrial application of chitosan with minimum cost but the highest HM removal efficiency.

The effect of Ca-bearing contents in chitosan on Pb 2+, Cd 2+ and Cu 2+ adsorption and its adsorption mechanism.

The preparation of chitosan has been investigated for more than half century; however, the application of chitosan for heavy metal (HM) adsorption is still under research. This study investigated the effects of chitosan with chemically controlled Ca-bearing contents (CBC) on Pb2+, Cd2+ and Cu2+ adsorption in the solution with the initial pH values of 2.10, 4.14 and 6.13. Highly purified chitosan showed the optimum HM adsorption at the initial pH values of 4.14 and 6.13, and the adsorption mechanism was chemisorption involving valence forces through sharing or exchange of electrons between the chitosan and HM ions. Highly purified chitosan prepared from HCl treated chitin only showed effective for Pb2+, however, those prepared from CH3COOH treated chitin showed effective for Pb2+, Cd2+ and Cu2+ adsorption due to a little amount of CBC. The HM adsorption mechanisms of partly purified chitosan were precipitation due to CBC and biosorption. Chitosan with 73% CBC showed the optimum adsorption of Pb2+ (755 mg/g) at an initial pH value of 2.10 while Cd2+ (979 mg/g) and Cu2+ (877 mg/g) at the initial pH values of 4.14 and 6.13. High Ca(OH)2-bearing chitosan prepared from HCl and H2SO4 treated chtin showed the optimum Cd2+ (978 mg/g) and Cu2+ (852 mg/g) adsorption at an initial pH value of 2.10. Biosorption isotherm and kinetics models showed that the adsorption data of Pb2+, Cd2+ and Cu2+ onto the surface of chitosan was well-fitted by Langmuir model and Pseudo-second-order model with correlation coefficient (R2 > 0.95 and R2 > 0.91, respectively). Pseudo-second-order model showed that the adsorption capacity strongly depended on CBC in chitosan and initial pH value of HM solution. It is concluded that the HM adsorption by the prepared chitosan is a chemical process that was supported by CBC of chitosan through elevating solution pH value.

Determination of the selected heavy metal and metalloid contents in various types of plastic bags.

The immense plastic bags (PBs) consumption is the potential threat for environment and public health due to discharging toxic pollutants (i.e., metals and metalloids) throughout its entire life cycle. The purpose of the study was to investigate the heavy metal (HM) contents in PE, HDPE, LDPE and PVC bags with several colors and applications. The HM contents in the PB were analyzed by ICP-OES after acid digestion. Results demonstrated that the HM contents in PB notably disagreed based on the used polymer types, colors and applications. The highest contents of Pb, Cr, Cd, As, Cu and Zn were 66, 75, 16, 28, 96 and 154 in mg/kg unit, respectively in PE bags; 71, 74, 34, 39, 430 and 212 in mg/kg unit, respectively in HDPE bags; 12, 74, 23, 43, 158 and 54 in mg/kg unit, respectively in LDPE bags; and 16, 23, 474, 12, 45 and 90 in mg/kg unit, respectively in PVC bags. Furthermore, 12 out of 36 PB samples were noticed to violate the standards (ISO 8124-3) in terms of Pb, Cr, Cd and As contents. So there is the potentiality of HM and metalloid pollution from discarded PB during treatment, recycling and disposal.