An interpretation to Cr(Ⅵ) leaching concentration rebound phenomenon with time in ferrous-reduced Cr(Ⅵ)-bearing solid matrices

The occurrence of "yellowing" phenomenon and its main driving factors after the remediation of chromium (Cr)-contaminated soils: A literature review

Chromium (Cr) is a highly toxic element, which is widely present in environment due to industrial activities. One of most applicable technique to clean up Cr pollution is chemical reduction. However, the Cr(VI) concentration in soil increases again after remediation, and meanwhile the yellow soil would appear, which is commonly called as "yellowing" phenomenon. To date, the reason behind the phenomenon has been disputed for decades. This study aimed to introduce the possible "yellowing" mechanism and the influencing factors based on the extensive literature review. In this work, the concept of "yellowing" phenomenon was explained, and the most potential reasons include the reoxidation of manganese (Mn) oxides and mass transfer were summarized. Based on the reported finding and results, the large area of "yellowing" is likely to be caused by the re-migration of Cr(VI), since it could not sufficiently contact with the reductant under the effects of the mass transfer. In addition, other driving factors also control the occurrence of "yellowing" phenomenon. This review provides valuable reference for the academic peers participating in the Cr-contaminated sites remediation.

Fe-biochar for simultaneous stabilization of chromium and arsenic in soil: Rational design and long-term performance

Excess chromium (Cr) and arsenic (As) coexist in soil such as chromated copper arsenate (CCA) contaminated sites, leading to high risks of pollution. Fe-biochar with adjustable redox activity offers the possibility of simultaneous stabilization of Cr and As. Here, a series of Fe-biochar with distinct Fe/C structure were rationally produced for the remediation of Cr and As contaminated soil (BCX-Fe, X represented the biomass/Fe ratio). Adsorption tests showed that maximal adsorption of BC5-Fe for Cr(VI) and As(III) reached 73.7 and 81.3 mg/g. A 90-day soil remediation experiment indicated that the introduction of 3% (w/w) Fe-biochar reduced the leaching state of Cr(VI) by 93.8-99.7% and As by 75.2-95.6%. Under simulated groundwater erosion for 10 years and acid rain leaching for 7.5 years, the release levels of Cr(VI) and As in the BC5-Fe remediated soil could meet the groundwater class IV standard in China (Cr(VI)<0.1 mg/L, As<0.05 mg/L). Accelerated aging tests demonstrated that BC5-Fe had long-term Cr and As stabilization ability. The quenching experiment, EPR, and XPS suggested that the corrosion products of Fe dominated the adsorption and redox reactions, while the O groups acted as electron transfer stations and constituted redox microcirculation in the synchronous uptake of Cr/As. Based on these insights, we believe that our study will provide meaningful information about the application potential of Fe-biochar for the heavy metal contaminated soil remediation.

Evaluation of remediation of Cr(VI)-contaminated soils by calcium polysulfide: Long-term stabilization and mechanism studies

In the remediation of Cr(VI)-contaminated soils, the effectiveness and long-term stability are critical qualities for the selection of a reductant. In current engineering practices, iron-based materials and sulfides are the most prevalent reductants, and calcium polysulfide (CaS₄) is considered as the one with the highest effectiveness and strongest long-term stabilization ability. But this opinion is questioned by the high interference ability of CaS₄ to soil Cr(VI) analysis. This study provides a pretreatment method to eliminate the interference of residual ferrous and sulfides to soil Cr(VI) analysis. By this pretreatment method and comparing with FeSO₄ and Na₂S, the mechanisms of the false high effectiveness and strong long-term stabilization ability of CaS₄ is revealed. In the remediation process, CaS₄ produces much elemental sulfur (S⁰) which remains in the soils. During the alkaline digestion, the S⁰ generates polysulfide which reduces the extracted Cr(VI), inducing serious negative analysis bias. When this negative bias is eliminated by pretreatment method, analysis results show that CaS₄ exhibits lowest effectiveness. The S⁰ cannot be leached away from soils and oxidized by oxygen under natural conditions, this makes CaS₄ exhibit a persistent interference ability, which is mistaken for a strong long-term stabilization ability.

Microwave-enhanced reductive immobilization of high concentrations of chromium in a field soil using iron polysulfide

High concentrations of Cr(VI) are often detected in contaminated soil. Yet, cost-effective remediation technologies have been lacking. In this study, we prepared a type of FeS_x based on commercial FeSO₄.7H₂O and CaS_x and tested a microwave-assisted technology based on FeS_x for reductive immobilization of high concentrations of Cr(VI) in a field contaminated soil. The as-prepared FeS_x particles appeared as a honeycomb-like and highly porous structure. The microwave-assisted FeS_x reduction process was able to rapidly reduce the TCLP-based reachability of Cr(VI) from 391.8 to 2.6 mg·L^-1. The dosage of FeS_x, S/Fe molar ratio, initial moisture content, microwave power, and irradiation time can all affect the treatment effectiveness. After 500 days curing under atmospheric conditions, the TCLP-leached concentration of Cr remained below the regulatory limit of 5 mg·L^-1, while other treatments failed to meet the goal. S_x^2- or S^2- served as the primary electron donors, whereas Fe facilitated the microwave absorption and the formation of the stable final product of FeCr₂O₄. S and Fe are mostly precipitated in soil. The microwave-assisted FeS_x reduction was shown to be an effective approach to rapidly reduce the leachability of Cr(VI) in contaminated soil, especially in heavily contaminated soil.

Interference of sulfide with iron ions to the analysis of Cr(Ⅵ) by Method 3060a & Method 7196a

When Cr(Ⅵ)-contaminated soil was remediated with sulfide, due to incomplete reaction and overdose, excess sulfide could remain in the soil inducing unacceptable underestimation of residual Cr(Ⅵ) analysis, which would get worse when iron ions were introduced in. Unfortunately, the quality control policy of Method 3060a cannot distinguish the difference between real zero and false zero residual Cr(Ⅵ) when their spike recoveries are zero. This paper systematically investigated the sulfide-induced Cr(Ⅵ) reduction in Cr(Ⅵ) analysis by Method 3060a & Method 7196a. Experimental results indicate that the sulfide-Cr(Ⅵ) reaction mainly occurs during alkaline digestion and pH adjustment processes, and iron ions as an electron transfer carrier between sulfide and Cr(Ⅵ) can catalyze the redox reaction during alkaline digestion. Besides, the high temperature in alkaline digestion significantly accelerates the redox reaction which usually is negligible at room temperature. Sulfur of high concentrations in remediated soils can also cause unacceptable underestimation of Cr(Ⅵ) due to the production of sulfide ions by disproportionation reaction in alkaline digestion. This paper also provides a method to eliminate sulfide ions from alkaline digestion solution before pH adjustment and suggests some possible solutions to the ferrous or sulfide-induced Cr(Ⅵ) analysis bias.

Research on the characterization, reactivity, and transportability of porous silicon-coated nanoscale zero-valent iron

In practical conditions, the remediation efficiency is always very limited due to the rapid aggregation and deactivation of nanoscale zero-valent iron (nZVI). Porous SiO2-coated technology can effectively suppress the agglomeration and oxidation of nZVI particle, resulting in the excellent dispersion and stability in water. A series of characterization results show that the porous SiO2-coated Fe0 (Fe0@p-SiO2) was a core-shell structure composite, with Fe0 as the core and the porous SiO2 as the shell. Moreover, the prepared composite material has a large specific surface area (244.04 m2/g). The experiments of nitrobenzene (NB) reduction and one-dimensional simulation column indicated that the different amounts of NaOH in the preparation process lead to the different structures, shapes, and particle sizes of prepared composite materials, which have significant effects on its activity and transportability. Under the conditions investigated, the optimum ratio of Fe0@p-SiO2 synthesis was nFe3+:n(Tetraethoxy silane, TEOS):nNaOH = 1:1.85:1.19, and the corresponding reduction efficiency of NB to aniline (AN) and maximum normalized outflow concentration (Cmax/C0) was 100% and 0.79, respectively. The SiO2-coated technology gives nZVI preparation greater control over the structure, shape, and particle size of modified nZVI composite, which has great potential in in situ remediation of groundwater contamination.

Wheat Straws and Corn Straws as Adsorbents for the Removal of Cr(VI) and Cr(III) from Aqueous Solution: Kinetics, Isotherm, and Mechanism

In this paper, the adsorption properties of wheat straw (WS) and corn straw (CS) for Cr(VI) and Cr(III) in solution were studied. The effects of adsorption time, pH of the solution, temperature, and initial concentration of metal ions on adsorption capacity were investigated. The adsorption mechanism was discussed. The results showed that the adsorption isotherms of WS and CS for Cr(VI) and Cr(III) satisfied the Langmuir equation. By fitting the Langmuir equation, the saturated adsorption capacity of WS for Cr(VI) and Cr(III) can reach 125.6 and 68.9 mg g^-1, and that of CS for Cr(VI) and Cr(III) can reach 87.4 and 62.3 mg g^-1 , respectively. The adsorption kinetics conformed to the pseudo-second-order kinetic equation. The effect of temperature on the adsorption capacity was not significant. Physical diffusion and chemical adsorption coexist in the process of adsorption of metal ions by straws, and chemical adsorption is dominant, and the effect of physical diffusion on the chemical adsorption rate can be neglected. It can be seen from the experimental results that the treatment of chromium-containing wastewater by using cheap and easily available wheat straw and corn straw had a remarkable effect. The adsorbed straw could be completely desorbed and had excellent recyclability, indicating that the straws are ideal adsorbents.