Probing reactivity sites on pyrite-oxidative interactions with 4-chlorophenol

Influence of natural cellulose on hydroxyl radical generation by abiotic oxidation of pyrite under acidic conditions

Natural cellulose is one of the most important substances coexisting on the surface of pyrite. Oxidation of pyrite produces hydroxyl radicals (•OH). In this study, a pyrite-cellulose binary system was constructed with natural cellulose to investigate the effect of cellulose on the mechanism of •OH generation via oxidation of pyrite, and the mechanism for abiotic oxidative •OH production by pyrite under the influence of cellulose was investigated with oxidation and quenching experiments and characterization techniques. It was demonstrated that cellulose was chemisorbed onto the pyrite surface and some of the Fe(II) on the pyrite surface was masked, thus inhibiting the reaction between pyrite and O2 and decreasing the •OH production level from 33.54 to 22.48 μM under oxic conditions. In addition, the cellulose caused SS bond breakage, resulting in defects on the pyrite surface, which oxidized H2O to produce •OH in anoxic conditions. Therefore, under anoxic conditions, cellulose promoted the production of •OH and increased the •OH content from 11.85 to 14.78 μM. In addition to •OH, pyrite oxidation also produced SO42-. The amount of SO42- produced by a single pyrite crystal was less than that produced in the pyrite-cellulose system in all cases, and the amount produced under oxic conditions was approximately 10 times greater than that produced under anoxic conditions. More sulfate production indicated more sulfur intermediates during the reaction, such as S2O32-, which may decompose to produce S0 or Sn2- adsorbed on pyrite and decrease the amount of •OH produced. During the oxidation of pyrite by H2O2, cellulose competed with pyrite to react with H2O2, which inhibited the reaction between pyrite and H2O2 and decreased •OH production. Therefore, natural cellulose influenced the abiotic oxidation of pyrite to produce •OH.

2D FeSx Nanosheets by Atomic Layer Deposition: Electrocatalytic Properties for the Hydrogen Evolution Reaction

2-dimensional FeS_x nanosheets of different sizes are synthesized by applying different numbers of atomic layer deposition (ALD) cycles on TiO₂ nanotube layers and graphite sheets as supporting materials and used as an electrocatalyst for the hydrogen evolution reaction (HER). The electrochemical results confirm electrocatalytic activity in alkaline media with outstanding long-term stability (>65 h) and enhanced catalytic activity, reflected by a notable drop in the initial HER overpotential value (up to 26 %). By using a range of characterization techniques, the origin of the enhanced catalytic activity was found to be caused by the synergistic interplay between in situ morphological and compositional changes in the 2D FeS_x nanosheets during HER. Under the application of a cathodic potential in alkaline media, the as-synthesized 2D FeS_x nanosheets transformed into iron oxyhydroxide-iron oxysulfide core-shell nanoparticles, which exhibited a higher active catalytic surface and newly created Fe-based HER catalytic sites.

Role of complexing agents on oxidative degradation of chlorophenolic compounds by pyrite-Fenton process: Batch and column experiments

This study involves batch reactor and fixed-bed continuous flow experiments to determine the effects of complexing agents (e.g., tartrate and citrate) on the treatment of chlorophenolic (CP) compounds using heterogeneous Fenton system with pyrite mineral as the iron source. While the addition of organic ligands to the batch systems adversely affected CP removal, organic ligands had a beneficial effect on CP removal in column systems. Although the ligands extended the life span of pyrite-packed columns by removing surface oxidation products through the formation of soluble Fe-ligand complexes, the ligands competed against CPs for hydroxyl radicals (HO^*). The competitive effect was much higher in batch systems since pyrite loading was very low in order to generate sufficient hydroxyl radicals. On the other hand, at much higher pyrite loading of column experiments, the HO^* radicals generated during Fenton process were sufficient to overcome the competitive effect exerted by organic ligands. In spite of much higher Fe solubility in the presence of citrate, citrate was less effective in enhancing CP removal in column systems compared to tartrate since the competitive effect caused by citrate for HO^* radicals was more than that exerted by tartrate.

Oxidative degradation of chlorophenolic compounds with pyrite-Fenton process

Batch experiments, in conjunction with chromatographic and spectroscopic measurements, were performed to comparatively investigate the degradation of various chlorophenolic (CP) compounds (e.g., 2-CP, 4-CP, 2,3-DCP, 2,4-DCP, 2,4,6-TCP, 2,3,4,6-TeCP) by a modified Fenton process using pyrite as the catalyst. The batch results show that the CP removal by pyrite-Fenton process was highly dependent on chemical conditions (e.g., pH, CP and pyrite concentration), CP type, number and location of chlorine atoms on the aromatic ring. With the exception of 2,3,4,6-TeCP and 2,3-DCP, the CP removal decreased with increasing the number of chlorine constituents. While the main mechanism responsible for monochlorophenol removal (e.g., 2-CP and 4-CP) was the hydroxyl radical attack on aromatic rings, the CP removal for multichlorophenolic compounds (e.g., 2,3,4,6-TeCP) was driven by both: (1) hydroxyl radical attack on aromatic rings by both solution and surface-bound hydroxyl radicals and (2) adsorption onto pyrite surface sites. The adsorption affinity increased with increasing the number of Cl atoms on the aromatic ring due to enhanced hydrophobic effect. The TOC removal was not 100% complete for all CPs investigated due to formation of chemically less degradable chlorinated intermediate organic compounds as well as low molecular weight organic acids such as formic and acetic acid. Spectroscopic measurements with SEM-EDS, zeta potential and XPS provided evidence for the partial oxidation of pyrite surface Fe(II) and disulfide groups under acidic conditions.

Removal of metals from lead-zinc mine tailings using bioleaching and followed by sulfide precipitation

Mine tailings often contain significant amounts of metals and sulfide, many traditional operations used to minerals was not as good as those currently available. This study investigated metals removal from lead-zinc mine tailings using bioleaching and followed by sulfide precipitation. Metals were dissolved from the tailings by the bacteria in a bioleaching reactor. During a 10% pulp density bioleaching experiment, approximately 0.82% Pb, 97.38% Zn, and 71.37% Fe were extracted after 50 days. With the pulp density of 10% and 20%, the dissolution of metals followed shrinking core kinetic model. Metals (Pb, Zn, and Fe) present in the pregnant bioleaching leachate. Metals were next precipitated as a sulfide phase using sodium sulfide (Na₂S). Metal precipitations were selectively and quantitatively produced from the bioleaching leachate by adding Na₂S. More than 99% of the zinc and 75% of the iron was precipitated using 25 g/L Na₂S in the bioleaching leachate. The results in the study were to provide useful information for recovering or removing metals from lead-zinc mine tailings.

Pyrite nanoparticles as a Fenton-like reagent for in situ remediation of organic pollutants

The Fenton reaction is the most widely used advanced oxidation process (AOP) for wastewater treatment. This study reports on the use of pyrite nanoparticles and microparticles as Fenton reagents for the oxidative degradation of copper phthalocyanine (CuPc) as a representative contaminant. Upon oxidative dissolution in water, pyrite (FeS2) particles can generate H2O2 at their surface while simultaneously promoting recycling of Fe(3+) into Fe(2+) and vice versa. Pyrite nanoparticles were synthesized by the hot injection method. The use of a high concentration of precursors gave individual nanoparticles (diameter: 20 nm) with broader crystallinity at the outer interfaces, providing a greater number of surface defects, which is advantageous for generating H2O2. Batch reactions were run to monitor the kinetics of CuPc degradation in real time and the amount of H2O2. A markedly greater degradation of CuPc was achieved with nanoparticles as compared to microparticles: at low loadings (0.08 mg/L) and 20 h reaction time, the former enabled 60% CuPc removal, whereas the latter enabled only 7% removal. These results confirm that the use of low concentrations of synthetic nanoparticles can be a cost effective alternative to conventional Fenton procedures for use in wastewater treatment, avoiding the potential risks caused by the release of heavy metals upon dissolution of natural pyrites.