Successive Reduction Cycles Research Articles

Magnetic hybrids synthesized from agroindustrial byproducts for highly efficient removal of total chromium from tannery effluent and catalytic reduction of 4-nitrophenol

The use of industrial waste to synthesize materials of technological interest is a rational way to minimize or solve environmental pollution problems. This work reports the removal of chromium ions from tannery effluent using magnetic hybrid adsorbents synthesized from waste biomass: coconut mesocarp (HCN), sugar cane bagasse (HBS), sawdust (HS), and termite nest (HT). The presence of organic matter and the cobalt ferrite phase in the structures of the materials were confirmed by XRD and FTIR analyses. Removal assays performed at different pH values showed the effectiveness of the adsorbents for removal at the natural pH of the effluent (pH 4.75), with adsorption capacity exceeding 6.6 mg g−1 for all the hybrids studied. The adsorption processes showed fast kinetics, with the HS hybrid removing 5.1 mg g−1 of the chromium species present in the effluent in the first 5 min. The HS hybrid presented the highest removal capacity, 6.7 mg g−1, while HBS showed the lowest, 6.6 mg g−1. The removal at equilibrium showed the following increasing order of efficiency of the hybrids: HBS < HCN < HT ≈ HS. The HCN and HT adsorbent matrices saturated with chromium ions (HCNCr and HTCr) showed excellent catalytic performance in the reduction of 4-nitrophenol, with 99.9% conversion within 180–240 s. The other hybrids did not present catalytic activity. The materials showed high capacities for reuse in two successive reduction cycles. The findings highlight the effectiveness of an industrial symbiosis approach to the development of new technologically important materials.

Applications of magnetic hybrid adsorbent derived from waste biomass for the removal of metal ions and reduction of 4-nitrophenol

The use of industrial waste to synthesize materials of technological interest is a rational way to minimize or solve environmental pollution problems. This work investigates the adsorption of cadmium and lead ions by magnetic hybrid adsorbents synthesized using the in natura biomasses coconut mesocarp (CCFe), sawdust (SAFe), and termite nest (TEFe) for the organic phases and magnetic cobalt ferrite as the inorganic phase. The formation of a cobalt ferrite phase was confirmed by XRD. The use of XRD and FTIR analyses revealed the presence of organic matter in the structure of the material. Removal assays performed at different pH values (2.0–8.0) showed the effectiveness of the adsorbent for the removal of Pb2+ at pH 3.0 and Cd2+ at pH 4.0. The adsorption processes showed fast kinetics, with removal of 79–86% of Pb2+ and 49% of Cd2+ within only 5 min, and removal of 92–96% of the metal species at equilibrium. In the case of cadmium, the hybrid sorbents (CCFe, SAFe, and TEFe) showed high removal capacity after three reuse cycles, while the removal of lead decreased from 99% to 40%. The adsorbent matrices saturated with the recovered cadmium and lead ions showed excellent catalytic performance in the reduction of 4-nitrophenol, with 99.9% conversion within 43–56 s. The materials showed high capacities for reuse in three successive reduction cycles. The findings highlight the effectiveness of an industrial symbiosis approach to the development of new technologically important materials.

Evaluation of activated carbon fiber supported nanoscale zero-valent iron for chromium (VI) removal from groundwater in a permeable reactive column

An activated carbon fiber supported nanoscale zero-valent iron (ACF-nZVI) composite for Cr(VI) removal from groundwater was synthesized according to the liquid phase reduction method. The techniques of N2 adsorption/desorption, FESEM, EDX, XRD and XPS were used to characterize the ACF-nZVI composite and the interaction between the ACF-nZVI composite and Cr(VI) ions. Batch experiments were conducted to evaluate the effects of several factors, including the amount of nZVI on activated carbon fiber (ACF), pH value, initial Cr(VI) concentration, and co-existing ions on Cr(VI) removal. The results indicate that presence of ACF can inhibit the aggregation of nanoscale zero-valent iron (nZVI) particles and increase its reactivity, and the Cr(VI) removal efficiency increases with increasing amounts of nZVI on ACF and a decrease in the initial Cr(VI) concentration. In acidic conditions, almost 100% of Cr(VI) in solution can be removed after 60 min of reaction, and the removal efficiency decreases with increasing initial pH values. The Cr(VI) removal is also dependent on the co-existing ions. Reusability experiments on ACF-nZVI demonstrate that the ACF-nZVI composite can keep a high reactivity after five successive reduction cycles. The removal mechanisms are proposed as a two-step interaction including the physical adsorption of Cr(VI) on the surface or inner layers of the ACF-nZVI composite and the subsequent reduction of Cr(VI) to Cr(III) by nZVI.