Preparation and characterization of higher degree-deacetylated chitosan-coated magnetic adsorbent for the removal of chromium(VI) from its aqueous mixture

Modified magnetic chitosan materials for heavy metal adsorption: a review

Magnetic chitosan materials have the characteristics of both chitosan and magnetic particle nuclei, showing the characteristics of easy separation and recovery, strong adsorption capacity and high mechanical strength, and have received extensive attention in adsorption, especially in the treatment of heavy metal ions. In order to further improve its performance, many studies have modified magnetic chitosan materials. This review discusses the strategies for the preparation of magnetic chitosan using coprecipitation, crosslinking, and other methods in detail. Besides, this review mainly summarizes the application of modified magnetic chitosan materials in the removal of heavy metal ions in wastewater in recent years. Finally, this review also discusses the adsorption mechanism, and puts forward the prospect of the future development of magnetic chitosan in wastewater treatment.

Advances in Sorptive Removal of Hexavalent Chromium (Cr(VI)) in Aqueous Solutions Using Polymeric Materials

Sorptive removal of hexavalent chromium (Cr(VI)) bears the advantages of simple operation and easy construction. Customized polymeric materials are the attracting adsorbents due to their selectivity, chemical and mechanical stabilities. The mostly investigated polymeric materials for removing Cr(VI) were reviewed in this work. Assembling of robust functional groups, reduction of self-aggregation, and enhancement of stability and mechanical strength, were the general strategies to improve the performance of polymeric adsorbents. The maximum adsorption capacities of these polymers toward Cr(VI) fitted by Langmuir isotherm model ranged from 3.2 to 1185 mg/g. Mechanisms of complexation, chelation, reduction, electrostatic attraction, anion exchange, and hydrogen bonding were involved in the Cr(VI) removal. Influence factors on Cr(VI) removal were itemized. Polymeric adsorbents performed much better in the strong acidic pH range (e.g., pH 2.0) and at higher initial Cr(VI) concentrations. The adsorption of Cr(VI) was an endothermic reaction, and higher reaction temperature favored more robust adsorption. Anions inhibited the removal of Cr(VI) through competitive adsorption, while that was barely affected by cations. Factors that affected the regeneration of these adsorbents were summarized. To realize the goal of industrial application and environmental protection, removal of the Cr(VI) accompanied by its detoxication through reduction is highly encouraged. Moreover, development of adsorbents with strong regeneration ability and low cost, which are robust for removing Cr(VI) at trace levels and a wider pH range, should also be an eternally immutable subject in the future. Work done will be helpful for developing more robust polymeric adsorbents and for promoting the treatment of Cr(VI)-containing wastewater.

Enhanced microbial reduction of aqueous hexavalent chromium by Shewanella oneidensis MR-1 with biochar as electron shuttle

Biochar, carbonaceous material produced from biomass pyrolysis, has been demonstrated to have electron transfer property (associated with redox active groups and multi condensed aromatic moiety), and to be also involved in biogeochemical redox reactions. In this study, the enhanced removal of Cr(VI) by Shewanella oneidensis MR-1(MR-1) in the presence of biochars with different pyrolysis temperatures (300 to 800 °C) was investigated to understand how biochar interacts with Cr(VI) reducing bacteria under anaerobic condition. The promotion effects of biochar (as high as 1.07~1.47 fold) were discovered in this process, of which the synergistic effect of BMBC700(ball milled biochar) and BMBC800 with MR-1 was noticeable, in contrast, the synergistic effect of BMBCs (300-600 °C) with MR-1 was not recognized. The more enhanced removal effect was observed with the increase of BMBC dosage for BMBC700+MR-1 group. The conductivity and conjugated O-containing functional groups of BMBC700 particles themselves has been proposed to become a dominant factor for the synergistic action with this strain. And, the smallest negative Zeta potential of BMBC700 and BMBC800 is thought to favor decreasing the distance from microbe than other BMBCs. The results are expected to provide some technical considerations and scientific insight for the optimization of bioreduction by useful microbes combining with biochar composites to be newly developed.

Feasibility of magnetic nano adsorbent impregnated with activated carbon from animal bone waste: Application for the chromium (VI) removal

Chromium is the heavy metal which existing in the effluents cause extensive discomfort to the environmental components. Bone waste is widely generated in food processing industries and restaurants. It is now used by switching into activated carbon. By co-precipitation procedure, the activated carbon is coated with a nano adsorbent. The Fe₃O₄-BAC adsorbent potential was established in this study via several batch tests. The adsorbing adverts had super magnetic behavior, and the magnetization value was 22 emu/g. The SEM imaging of the Fe₃O₄-BAC shows an improved morphology of 100-446 nm, and the nanoparticles were monodispersed. The present investigation also delves into the study of system parameters on the removal of metal ions. Optimal adsorption has been found at the acidic pH at the contact time of 60min for the adsorbent amount of 5 g/L. Maximum capacity of adsorption by nano adsorbent was 27.86 mg/g. Thermodynamic and isotherm tests have established the process viability. In addition, the kinetic studies establish the inclination of the studied Fe₃O₄-BAC towards pseudo-first order models. Hence, Fe₃O₄-BAC could be potential adsorbent to remove chromium from an aqueous solution.

Preparation and Characterization of Chitosan/Bentonite Composites for Cr (VI) Removal from Aqueous Solutions

Chitosan/bentonite composites (CSBT) prepared by physical gelation were tested for the adsorption of Cr (VI) from aqueous solutions in this work. The composites were prepared at a mass ratio from 2 : 1 to 1 : 2, and a composite of 1 : 1 was found to be most suitable for efficient Cr (VI) removal. The influencing parameters, including temperature, adsorbent dose, and pH, were statistically optimized using response surface methodology (RSM) for the removal of Cr (VI). The pH was found to be the limiting factor during the adsorption process, and under the optimal conditions, namely, adsorbent dose of 400 mg/L, $\text{pH}=3$ , and temperature of 298 K, 87.61% Cr (VI) would be removed expectantly. The mechanism of Cr (VI) removal by CSBT was discussed, and the protonation of amino groups on chitosan followed by the combination of -NH3+ and anionic hexavalent chromium was the primary driving force. In addition, the removal of Cr (VI) onto CSBT was monolayer adsorption with a maximum adsorption capacity of 133.85 mg/g by the Langmuir isotherm. CSBT follows a pseudosecond-order kinetic model, and within 1.5 h, adsorption was observed to reach equilibrium. The calculated thermodynamic functions clarified that the adsorption process was exothermic and spontaneous below 312.60 K. CSBT could be regenerated after desorption by 0.5 mol/L NaOH solutions and exhibited superior reusability after six cycles. This study demonstrated composites of chitosan/bentonite as eco-friendly bioadsorbents for the removal of Cr (VI) from aqueous environments.

Adsorption process of tetrachloroethylene (PCE) on network growth ring

This study was undergone to investigate the adsorption characteristics of tetrachloroethylene (PCE) on water supply network growth ring. According to the chemical composition of real network growth ring, iron oxide mixture containing synthesized goethite and lepidocrocite was applied as simulated growth ring. The results demonstrate that competition could take place only between PCE and the co-present organic non-ionic compound, while inorganic salt had no discernible effect on PCE adsorption. A maximum adsorption capacity of 33.118 mg g^-1 at equilibrium was achieved. By the non-linear regression method, the equilibrium adsorption data fitted well with the Freundlich isotherm model (R² = 0.994), and the kinetic data obeyed pseudo-first order model (R² = 0.985). Thermodynamic tests indicate the spontaneous and exothermic nature of adsorption process. In addition, no significant variation between the FTIR spectra of the iron oxide mixture before and after adsorption was observed, which verifies that hydrogen bonds between PCE and mineral mixture could be neglected. An overview of the experimental results leads to the conclusion that the adsorption of PCE onto the simulate growth ring was driven by dispersion and hydrophobic interactions. As a case study, this work will provide some information about water supply securit protection.

Fabrication of terminal amino hyperbranched polymer modified graphene oxide and its prominent adsorption performance towards Cr(VI)

In order to increase the density and quantity of functional groups on adsorbent, three terminal amino hyperbranched polymer modified graphene oxide adsorbents including GO-HBP-NH₂-DETA, GO-HBP-NH₂-TETA and GO-HBP-NH₂-TEPA with N-containing functional group density of 7.21 wt%, 10.20 wt% and 12.43 wt%, respectively, were prepared and used for the adsorption and reduction of toxic hexavalent chromium Cr(VI) to less toxic Cr(III). The morphology and structure of obtained adsorbents were characterized by FT-IR, SEM, XRD, Raman, BET, XPS and zeta potential. The density of receptor sites (N_m) of the three adsorbents calculated from statistical physics model with one energy site were found to be 456.62, 604.54 and 636.03, respectively. Adsorption experiments demonstrated that the high adsorption capacities of the three adsorbents obtained from Langmuir isotherm model towards Cr(VI) were 245.01, 257.26 and 300.88 mg/g, respectively, suggesting that GO-HBP-NH₂-TEPA had better adsorption ability. The density functional theory calculation (DFT) indicated that GO-HBP-NH₂-TEPA was more likely to adsorb HCrO₄^- rather than Cr₂O₇^2- at lower pH conditions. A possible adsorption mechanism was also proposed where electrostatic interaction between Cr(VI) (HCrO₄^- or Cr₂O₇^2-) and the N functional group (+) on GO-HBP-NH₂-TEPA dominated the adsorption of Cr(VI) and reduction mechanism dominated the reduction of Cr(VI)-Cr(III).

Biochar as both electron donor and electron shuttle for the reduction transformation of Cr(VI) during its sorption

Biochar has been demonstrated to be a promising sorbent and its redox activity can be involved in environmentally relevant redox reactions. In this study, the electron transfer for the reduction transformation of Cr(VI) during its sorption by biochar were evaluated. Biochar derived from peanut shell at 350 °C could effectively remove Cr(VI) from solutions, accompanied by the reduction of Cr(VI) to Cr(III) which was more obvious at the strong acidity (pH = 2), compared to that at the weak acidity (pH = 4). The O-containing functional groups, e.g. CO and CO, were the electron donor moieties of biochar for the Cr(VI) reduction. Biochar could also act as electron shuttle, enhancing Cr(VI) reduction by lactate, especially at the weak acidity (pH = 4) where the reduction rates of Cr(VI) by lactate together with biochar were about 2 and 9 times higher than those by either biochar or lactate alone, respectively. The mediated electrochemical oxidation analysis showed that biochar could increase the electron donating capacity of lactate by up to 23 times at pH = 4. The O-centered radicals, e.g. semiquinone-type radicals, were related with biochar as electron shuttle, which was confirmed by the electron paramagnetic resonance analysis. As a result of electron acceptance from lactate and/or biochar, Cr(VI) was reduced into CrOOH evidenced by X-ray diffraction analysis. Our results indicated that biochar could act as both electron donor and electron shuttle for the reduction of Cr(VI) during the sorption process, making it an alternative for removal of toxic Cr(VI) from wastewaters.

Fabrication of Fe3O4@polydopamine@polyamidoamine core–shell nanocomposites and their application for Cu(ii) adsorption

Fe₃O₄@PDA@PAMAM nanocomposites were fabricated with a polydopamine assisted method, possessing excellent magnetic properties and high adsorption capacity for Cu(ii).