Effect of groundwater ions (Ca2+, Na+, and HCO3-) on removal of hexavalent chromium by Fe(II)-phosphate mineral

Groundwater Cr(VI) contamination and remediation: A review from 1999 to 2022

Hexavalent chromium (Cr(VI)) contamination of groundwater has traditionally been an environmental issue of great concern due to its bioaccumulative and highly toxic nature. This paper presents a review and bibliometric analysis of the literature on the interest area "Cr(VI) in groundwater" published in the Web of Science Core Collection from 1999 to 2022. First, information on 203 actual Cr(VI)-contaminated groundwater sites around the world was summarized, and the basic characteristics of the sources and concentrations of contamination were derived. 68.95% of the sites were due to human causes and 56.43% of these sites had Cr(VI) concentrations in the range of 0-10 mg/L. At groundwater sites with high Cr(VI) contamination due to natural causes, 75.00% of the sites had Cr(VI) concentrations less than 0.2 mg/L. A total of 936 papers on "Cr(VI) in groundwater" were retrieved for bibliometric analysis: interest in research on Cr(VI) in groundwater has grown rapidly in recent years; 59.4% of the papers were published in the field of environmental sciences. A systematic review of the progress of studies on the Cr(VI) removal/remediation based on reduction, adsorption and biological processes is presented. Out of 666 papers on Cr(VI) removal/remediation, 512, 274, and 75 papers dealt with the topics of reduction, adsorption, and bioremediation, respectively. In addition, several studies have demonstrated the potential applicability of natural attenuation in the remediation of Cr(VI)-contaminated groundwater. This paper will help researchers to understand and investigate methodological strategies to remove Cr(VI) from groundwater in a more targeted and effective manner.

Grinding siderite with ferric sulfate to generate an active ferrous source for Cr(VI) reduction

Activated siderite, endowed with excellent properties, was simply prepared by co-grinding with Fe sulfate to enhance its high reducing ability for Cr(VI). Batch experiments were conducted to investigate the main affecting parameters, such as material ratio, pH, temperature, etc. The removal of Cr(VI) by activated siderite was completed within 4 h of the reaction. The activated siderite maintained a high removal effect of Cr(VI) within a wide pH range (3-9). Various analytical methods, including XRD, SEM/EDS, XPS, etc., were employed to characterize the samples and discover variations before and after the reaction. The Fe (Ⅱ) in activated siderite becomes highly active, and it can even be released from the solid phase in the mildly acidic liquid phase to efficiently reduce Cr(VI) and mitigate its toxicity. These findings introduce an innovative approach for activating various minerals widely distributed in nature to promote the recovery of the ecological system.

Application of high-resolution techniques in the assessment of the mobility of Cr, Mo, and W at the sediment-water interface of Nansi Lake, China

There are few studies on the simultaneous behavior of chromium (Cr), molybdenum (Mo), and tungsten (W) belonging to group VIB of the periodic table. Herein, based on high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) technology, the vertical distribution characteristics of DGT-labile and soluble Cr, Mo, and W in two lakes of Nansi Lake (Weishan Lake and Dushan Lake) were analyzed. In addition, the net diffusion fluxes and R-value (CDGT/Csol) were used to evaluate the mobility and release risk of metals at the sediment-water interface. The results showed that the DGT-labile concentrations of the three metal elements (Cr, Mo, and W) in Weishan Lake were higher than those in Dushan Lake, both in overlying water and sediment. This is mainly due to the dredging of the Dushan Lake area, which can permanently remove the polluted sediment in the lake. Meanwhile, the exogenous input is relatively high near the tourist area of Weishan Island. The net diffusion fluxes indicate that the W has a potential release risk of diffusion to the overlying water in Dushan Lake. The release of Cr, Mo, and W is thought to be related to the reductive dissolution of Fe/Mn (hydr)oxides based on Pearson correlation coefficients. The R-values of Cr and W indicate that Cr and W belong to the partial continuity case. The R-value of Mo was lower than the minimum value, meaning that Mo belongs to the single diffusion type and it is difficult for Mo sediments to supply pore water.

Generation of Reactive Oxygen Species and Degradation of Pollutants in the Fe2+/O2/Tripolyphosphate System: Regulated by the Concentration Ratio of Fe2+ and Tripolyphosphate

Tripolyphosphate (TPP) has many advantages as a ligand for the optimization of the Fe²⁺/O₂ system in environmental remediation applications. However, the relationship between remediation performance and the Fe²⁺/TPP ratio in the system has not been previously described. In this study, we report that the degradation mechanism of p-nitrophenol (PNP) in Fe²⁺/O₂ systems is regulated by the Fe²⁺/TPP ratio under neutral conditions. The results showed that although PNP was effectively degraded at different Fe²⁺/TPP ratios, the results of specific reactive oxygen species (ROS) scavenging experiments and the determination of PNP degradation products showed that the mechanism of PNP degradation varies with the Fe²⁺/TPP ratio. When C_Fe²⁺ ≥ C_TPP, the initially formed O₂^•- is converted to •OH and the •OH degrades PNP by oxidation. However, when C_Fe²⁺ < C_TPP, the O₂^•- persists long enough to degrade PNP by reduction. Density functional theory (DFT) calculations revealed that the main reactive species of Fe²⁺ in the system include [Fe(TPP)(H₂O)₃]^- and [Fe(TPP)₂]^4-, whose content in the solution is the key to achieve system regulation. Consequently, by controlling the Fe²⁺/TPP ratio in the solution, the degradation pathways of PNP can be selected. Our study proposed a new strategy to regulate the oxidation/reduction removal of pollutants by simply varying the Fe²⁺/TPP ratio of the Fe²⁺/O₂ system.

Competitive inhibition of catalytic nitrate reduction over Cu-Pd-hematite by groundwater oxyanions

The presence of various oxyanions in the groundwater could be the main challenge for the successive application of Cu-Pd-hematite bimetallic catalyst to aqueous NO₃^- reduction due to the inhibition of its catalytic reactivity and alteration of product selectivity. The batch experiments showed that the reduction kinetics of NO₃^- was strongly suppressed by ClO₄^-, PO₄^3-, BrO₃^- and SO₃^2- at low concentrations (>5 mg/L) and HCO₃^-, CO₃^2-, SO₄^2- and Cl^- at high concentrations (20-500 mg/L). The presence of anions significantly changing the end-product selectivities influenced high N₂ selectivity. The selectivity toward N₂ increased from 55% to 60%, 60%, and 70% as the concentrations of PO₄^3-, SO₃^2-, and SO₄^2- increased, respectively. It decreased from 55% to 35% in the presence of HCO₃^- and CO₃^2- in their concentration range of 0-500 mg/L. The production of NO₂^- was generally not detected, while the formation of NH₄⁺ was observed as the second by-product. It was found that the presence of oxyanions in the NO₃^- reduction influenced the reactivity and selectivity of bimetallic catalysts by i) competing for active sites (PO₄^3-, SO₃^2-, and BrO₃^- cases) due to their similar structure, ii) blockage of the promoter and/or noble metal (HCO₃^-, CO₃^2-, SO₄^2-, Cl^- and ClO₄^- cases), and iii) interaction with the support surface (PO₄^3- case). The results can provide a new insight for the successful application of catalytic NO₃^- reduction technology with high N₂ selectivity to the contaminated groundwater system.

Removal and recycling of hexavalent chromium from alkaline wastewater via a new ferrite process to produce the valuable chromium ferrite

Current technologies for removal of Cr(VI) are generally fit for acidic wastewater. In this study, a new ferrite process for removal and recycling of Cr(VI) from alkaline wastewater to produce the valuable chromium ferrite has been developed. The results show that this new ferrite method is a one-step process which can be divided into two successive reactions including Cr(VI) reduction to form coprecipitation (Cr_0.25Fe_0.75(OH)₃) and subsequently magnetic conversion of Cr_0.25Fe_0.75(OH)₃ induced by Fe²⁺ under the same alkaline condition. The total Fe/Cr mole ratio of 5:1 is at least required for the chromium ferrite transformation. Increasing temperature and pH can enhance the interaction of Fe²⁺ with Cr_0.25Fe_0.75(OH)₃ and further promote the formation of chromium ferrite, while suppressing the generation of nonmagnetic by-product goethite. Almost pure chromium ferrite is formed under proposed optimum conditions (Fe/Cr = 7:1, 65 °C and pH of 9) with Cr(VI) removal ratio around 100%. The Cr(VI) remained in the filtrate can be reduced to 0.01 mg/L which is much lower than the limits concentration for surface water (≤0.05 mg/L). The chromium ferrite product whose molecular formula can be expressed as Cr_0.5-xFe_2.5+xO₄ (where 0 ≤ x < 0.5) presents good magnetic properties and has the potential to be recycled as a useful material.

Migration and fate of characteristic pollutants migration from an abandoned tannery in soil and groundwater by experiment and numerical simulation

The tannery industry is an integral part of economic development in many developing countries, and the environmental pollution caused by the tannery industry cannot be ignored. In this study, soil and groundwater samples at different depths were collected from an abandoned tannery to investigate the temporal and spatial distribution of characteristic pollutants produced by tanning. The concentrations of Cr, Cl, F and NH₄⁺-N in the soil from the sludge temporary storage area were higher than those from the liming and unhairing workshop, chrome tanning workshop, wastewater outlet, and around wastewater pond. The concentration of Cr(VI) in all sampling sites was below the detection limit. The main species of Cr in the groundwater were Cr(NH₃)₆Cl²⁺ and CrO₄^2- based on the simulation of Visual MINTEQ. The saturation index was negative and changed with time indicating that Cr existed in the dissolved phase. The proportion of Cr(VI) to total Cr was negatively correlated with the saturation index in village 1 and village 3. The simulation results from Visual MODFLOW and MT3DMS showed that the migration of Cr, NH₄⁺-N, Cl^- and F^- mainly occurred in the Quaternary system. The coverage of the pollution plume of pollutants in villages 1 and 3 was as follows: Cr > NH₄⁺-N > Cl^- > F^-. Two decay rate calculation methods of pollutants with migration time and distance were put forward to provide a basis for the actual investigation of the pollution migration scope and time determination.

Critical Review of Advances in Engineering Nanomaterial Adsorbents for Metal Removal and Recovery from Water: Mechanism Identification and Engineering Design

Nanomaterial adsorbents (NAs) have shown promise to efficiently remove toxic metals from water, yet their practical use remains challenging. Limited understanding of adsorption mechanisms and scaling up evaluation are the two main obstacles. To fully realize the practical use of NAs for metal removal, we review the advanced tools and chemical principles to identify mechanisms, highlight the importance of adsorption capacity and kinetics on engineering design, and propose a systematic engineering scenario for full-scale NA implementation. Specifically, we provide in-depth insight for using density functional theory (DFT) and/or X-ray absorption fine structure (XAFS) to elucidate adsorption mechanisms in terms of active site verification and molecular interaction configuration. Furthermore, we discuss engineering issues for designing, scaling, and operating NA systems, including adsorption modeling, reactor selection, and NA regeneration, recovery, and disposal. This review also prioritizes research needs for (i) determining NA microstructure properties using DFT, XAFS, and machine learning and (ii) recovering NAs from treated water. Our critical review is expected to guide and advance the development of highly efficient NAs for engineering applications.

Biosynthesis of vivianite from microbial extracellular electron transfer and environmental application

Vivianite (Fe₃(PO₄)₂·8H₂O) is a common hydrous ferrous phosphate mineral which often occurs in reductive conditions, especially anoxic non-sulfide environment containing high concentrations of ferrous iron (Fe²⁺) and orthophosphate (PO₄^3-). Vivianite is an important product of dissimilatory iron reduction and a promising route for phosphorus recovery from wastewater. Its formation is closely related to the extracellular electron transfer (EET), a key mechanism for microbial respiration and a crucial explanation for the reduction of metal oxides in soil and sediments. Despite of the natural ubiquity, easy accessibility and attractive economic value, the application value of vivianite has not received much attention. This review introduces the characteristics, occurrence and biosynthesis of vivianite from microbial EET, and systematically analyzes the application value of vivianite in the environmental field, including immobilization of heavy metals (HMs), dechlorination of carbon tetrachloride (CT), sedimentary phosphorus sequestration and eutrophication alleviation. Additionally, its potential functions as a slow-release fertilizer are discussed as well. In general, vivianite is expected to make more contributions to the future scientific research, especially the solution of environmental problems. Overcoming the lack of understanding and some technical limitations will be beneficial to the further application of vivianite in environmental field.