Immobilization of hexavalent chromium in soil and groundwater using synthetic pyrite particles

Definitive Review of Nanobiochar

Nanobiochar is an advanced nanosized biochar with enhanced properties and wide applicability for a variety of modern-day applications. Nanobiochar can be developed easily from bulk biochar through top-down approaches including ball-milling, centrifugation, sonication, and hydrothermal synthesis. Nanobiochar can also be modified or engineered to obtain "engineered nanobiochar" or biochar nanocomposites with enhanced properties and applications. Nanobiochar provides many fold enhancements in surface area (0.4-97-times), pore size (0.1-5.3-times), total pore volume (0.5-48.5-times), and surface functionalities over bulk biochars. These enhancements have given increased contaminant sorption in both aqueous and soil media. Further, nanobiochar has also shown catalytic properties and applications in sensors, additive/fillers, targeted drug delivery, enzyme immobilization, polymer production, etc. The advantages and disadvantages of nanobiochar over bulk biochar are summarized herein, in detail. The processes and mechanisms involved in nanobiochar synthesis and contaminants sorption over nanobiochar are summarized. Finally, future directions and recommendations are suggested.

Study on the removal effect and mechanism of calcined pyrite powder on Cr(VI)

Pyrite exhibits considerable potential as an adsorbent in wastewater treatment. However, few pyrite adsorbents are directly obtained from natural pyrite, as most are composite materials that require a complex preparation process. To develop a pyrite-based adsorbent with a simple preparation process, pyrite was processed by calcination at 400, 600, and 800 °C for 4 h and ball-milled into a fine powder. The adsorption properties of the pyrite powder were systematically explored. The calcined pyrite powder was characterized by SEM-EDS and XRD. The results revealed that the pyrite calcined at 600 °C exhibited excellent adsorption properties and was primarily composed of Fe7S8. The optimum conditions for Cr(VI) removal were a temperature of 45 °C, an adsorbent dosage of 1 g, an equilibration time of 60 min, and an initial pH of 3. Moreover, the calcined pyrite powder exhibited excellent reusability, and the Cr(VI) removal rate exceeded 65% after three cycles. The Cr(VI) adsorption on pyrite can be well described by the Freundlich model and pseudo-second-order kinetic equation. The calcination temperature is the main factor affecting the adsorption performance of pyrite. Therefore, the calcined pyrite powder is expected to be an excellent adsorbent for Cr(VI) in the wastewater treatment industry.

Interaction of Lead and Cadmium Reduced Cadmium Toxicity in Ficus parvifolia Seedlings

Potentially toxic elements (PTEs) pollution occurs widely in soils due to various anthropogenic activities. Lead (Pb) and cadmium (Cd) coexist in soil frequently, threatening plant growth. To explore the interaction effect between Pb and Cd in Ficus parvifolia and the response of plant physiological characteristics to Pb and Cd stress, we designed a soil culture experiment. The experiment demonstrated that Pb stress improved leaf photosynthesis ability, while Cd stress inhibited it. Furthermore, Pb or Cd stress increased malonaldehyde (MDA) content, but plants were able to reduce it by increasing antioxidant enzyme activities. The presence of Pb could alleviate Cd phytotoxicity in plants by inhibiting Cd uptake and accumulation as well as increasing leaf photosynthesis and antioxidant ability. Pearson correlation analysis illustrated that the variability of Cd uptake and accumulation between Pb and Cd stress was related to plant biomass and antioxidant enzyme activities. This research will offer a new perspective on alleviating Cd phytotoxicity in plants.

Sulfur defect and Fe(III) (hydr)oxides on pyrite surface mediate tylosin adsorption in lake water: effect of solution chemistry and dissolved organic matter

Pyrite affects the adsorption of tylosin (TYL) due to their coexistence in the lake system. As well as the reactivity groups of S-S-H, S-OH, and Fe-OH, defects also have the possibilities to influence the adsorption of organic contaminants. However, the role of these active sites in antibiotic adsorption on pyrite has not been deeply studied. Besides, pH, N, P, dissolved oxygen, and dissolved organic matter (DOM) fluctuate greatly in lake at different seasons, which may change the surface characteristics of pyrite. Hence, the adsorption of TYL on natural pyrite considered solution chemistry and DOM in lake water was explored in this study. The fitting results of the kinetic and isotherm models showed that the adsorption included physical and chemical interactions. The neutral initial solution pH was conductive to TYL adsorption owing to the combined result of electrostatic and cover of Fe-oxyhydroxide. NO₃^- and NH₄⁺ had no effect on TYL adsorption, whereas H₂PO₄^- promoted adsorption by forming flocculent Fe(H₂PO₄)₃ precipitates. The dissolved oxygen increased adsorption. This is due to the co-promotion of the pyrite oxidation by oxygen and sulfur defects. The Fe(II)-DOM complex caused by pyrite surface oxidation reduced the concentration of TYL in solution by gathering. Except for the surface charge, reactivity groups on pyrite significantly influenced the adsorption of TYL. The bond fracture of S-S resulted in sulfur defects that contributed to pyrite oxidation. As a result, Fe(III)/Fe(II) on the surface of pyrite or in solution produced a complex Fe(III)/Fe(II) with anions and DOM. In addition, Fe(III)-S on sulfur defects interacted with the O-H of TYL through hydrogen bonding. Furthermore, the Fe-O-C bond is formed by the interaction of C-OH on TYL and Fe(III) (hydr)oxides on the surface of pyrite. The study provides a deep insight into the effect of pyrite surface active sites on amphoteric antibiotic adsorption. It helps to understand antibiotic migration and interactions with widespread pyrite in the real environment.

Purification of Low-Concentration Carbonyl Sulfide by Red Mud-Based Adsorbent

Bauxite residue, also known as red mud (RM), is a kind of industrial solid waste with high alkali content, complex composition and difficult utilization. In this study, a new type of RM-based adsorbent was prepared by using polyethylene glycol modified RM and was used to remove low concentration of COS in flue gas. The optimum preparation conditions of adsorbent and the optimum technological parameters of COS adsorption purification were investigated. Under the optimal conditions, the adsorption efficiency of the new adsorbent exceeds 95%, and the COS adsorption capacity reaches 63.56 mg/m³. The characterization results showed that the main active components of the adsorbent were active alkali, FeOOH and Fe₃O₄, and the main products were Na₂S₂O₃, Na₂SO₄, FeS and FeS₂.

Reductive immobilization of 99Tc(VII) by FeS2: The effect of marcasite

Reductive immobilization of ⁹⁹Tc by a synthetic FeS₂ mixture, i.e. marcasite-pyrite 60:40, was studied by a combined approach of batch experiments and powder X-ray diffraction, X-ray photoelectron spectroscopy as well as Raman microscopy. It was found that the FeS₂ mixture removes 100% of Tc from the suspension after 7 days in contact at 6.0 < pH ≤ 9.0. The retention outside that pH range was slower and incomplete. Spectroscopic analysis showed that the redox active species at pH 6.0 is Fe²⁺ as expected from previous works with pyrite. However, at pH 10.0 the surprising oxidation of S^2- to SO₄^2- was found responsible for Tc immobilization. This was explained by the high reactivity of marcasite that is easily oxidized to produce H₂SO₄. Our work provides new molecular insights into the reductive mobilization of Tc(VII) by oxidative formation of sulfate. The assigned molecular reactions may also be relevant for the assessment of other redox reactive contaminants. Technetium re-oxidation experiments showed that the fast oxidation of marcasite is associated to the reduction of the remaining Tc(VII) in solution, which gives marcasite the potential of Tc natural remediation since it delays the re-oxidation of Tc(IV).

Reductive materials for remediation of hexavalent chromium contaminated soil - A review

Chemical reduction of Cr(VI) to Cr(III) by reductive materials is the most widely used technology for the remediation of Cr(VI)-contaminated soil due to its high efficiency, adaptability and low cost. This paper reviews chromium chemistry and the materials that can effectively reduce Cr(VI) to Cr(III) for the remediation of Cr(VI)-contaminated soil, namely iron-bearing reductants, sulfur-based compounds and organic amendments. Moreover, we discuss the corresponding mechanisms involved in the process of immobilization of Cr(VI) in polluted soil, and emphasize the relationship between the materials remediation performance and soil environmental conditions. Besides, perspectives on the potential future researches of novel materials design and technological development in the remediation of Cr(VI) contaminated soil are also put forward.

Simultaneous removal of Cr(VI) and acid orange 7 from water in pyrite-persulfate system

As the industries advances at a fast pace, efficient and simultaneous removal of both heavy metals and organics from aqueous is essential to protecting public human health and environment. In this work, we used pyrite as reductant and catalyst for simultaneously reducing Cr(VI) and activating persulfate (PS) to degrade acid orange 7 (AO7). The results indicated that the simultaneous removal rate of AO7 and Cr(VI) by pyrite-PS was up to 100% within 60 min under acidic conditions. However, There was a competitive relationship between PS activation and Cr(VI) reduction for robbing Fe²⁺. At beginning of the reaction, the limited Fe²⁺ firstly activated persulfate rather than reduce Cr(VI). The effect of dosage of pyrite and PS on Cr(VI) reduction was more significant than that on AO7 degradation. Increased pyrite dosages from 1g·L^-1 to 6 g L^-1 resulted in enhanced Cr(VI) removal, and excessive PS (more than 0.4 g L^-1) was not beneficial to Cr(VI) removal. Electron paramagnetic resonance (EPR) spectroscopy and radical scavenger studies demonstrated that sulfate (SO₄^-·), singlet oxygen (¹O₂) and superoxide radical (·O₂^-) were the crucial reactive oxygen species (ROS) in the pyrite-PS system rather than hydroxyl radical (·OH). This study showed that the pyrite-PS system could simultaneously remove AO7 and Cr(VI), which provided a new idea for the actual wastewater treatment.

Immobilization of perrhenate using synthetic pyrite particles: Effectiveness and remobilization potential

Radioactive pertechnetate (TcO4^-) has been detected in nuclear waste affected soil and groundwater, posing significant effect on human health and the environment. Yet, cost-effective remediation of Tc-contaminated soil and groundwater remains challenging. To address this critical technology need, we prepared a class of pyrite (FeS₂) particles for effective immobilization of pertechnetate. Using perrhenate (ReO₄^-) as a non-radioactive surrogate of TcO₄^-, we tested the immobilization effectiveness of the material through batch kinetic experiments, and evaluated the remobilization potential of immobilized Re under anoxic (sealed from air) and oxic (exposed to air) conditions and in the presence of humic acid (HA), EDTA, nitrate, and a Chinese loess soil. The results showed that more acidic pH gave faster Re(VII) removal due to more abundant electron sources (Fe²⁺ and S₂^2-). X-ray diffraction (XRD) and/or X-ray photoelectron spectroscopy (XPS) analyses confirmed formation of ReO₂/ReS₂ as the major reduction products. The immobilized Re remained highly stable when aged for 360 days under anoxic conditions at different influence factors. Yet, the immobilized Re was vulnerable to oxygen oxidation, and about 78% of Re was remobilized after 40 days of exposure to air regardless of the initial pH (3.5-9.0) due to excessive pyrite oxidation and the associated pH drop (~2). HA at 120 mg/L inhibited Re remobilization under oxic conditions, which lowered the Re remobilization by ~21% after 40 days of oxic aging. The presence of EDTA facilitated dissolution of Fe but inhibited the dissolution of Re under oxic conditions. Nitrate showed negligible effect on Re remobilization. The presence of a Chinese loess soil effectively inhibited Re remobilization under both oxic and anoxic conditions, lowering the leachable Re by ~32% under oxic conditions. The findings may guide engineered application of pyrite particles as a long-lasting reducing material for immobilization pertechnetate or similar redox-active contaminants in soil and water.