Hexavalent chromium has aroused a series of environmental concerns due to its high mobility and toxicity. Iron and manganese oxides usually coexist in the environments and influence the speciation and geochemical cycling of chromium. However, the interaction mechanism of iron-manganese oxides with dissolved Cr(VI) remains largely unknown. In this work, the interaction processes of dissolved Cr(VI) and manganite in the presence of goethite coating were investigated, and the effects of pH (2.0–9.0) and iron oxide content were also studied. Manganite-goethite composites were formed with uniform micromorphologies in the system of manganite and Fe(II). In the reaction system of single manganite and Cr(VI), manganite could only adsorb but not reduce Cr(VI), with the adsorption amount decreasing at higher pHs. In the reaction system of manganite-goethite composites and Cr(VI), adsorbed Cr(VI) was reduced to Cr(III) by Fe(II) on composites surface. The generated Cr(III) was then retained as Cr(OH)₃ on the mineral surface. Goethite coating suppressed the re-oxidation of newly formed Cr(III) by manganite. The amounts of adsorbed Cr(VI) and generated Cr(III) increased with increasing iron oxide content, and increased first and then decreased with increasing pH. The Cr(III) formation and Cr(VI) adsorption amount reached the maximum at pH 5.0–6.0. The present work highlights the transformation and retention of Cr(VI) by iron-manganese oxides and provides potential implications for the use of such oxides in the remediation of Cr(VI) polluted waters and soils.

A microbially facilitated approach was developed to degrade 2, 2′, 4, 4′-tetrabrominated diphenyl ether (BDE-47). This approach consisted of biological production of Fe(II) and H₂O₂ by the dissimilatory metal-reducing bacterium Shewanella oneidensis MR-1 during the repetitive anoxic/oxic cycles and abiotic production of hydroxyl radical (HO●) with the biologically produced Fe(II) and H₂O₂ via Fenton reaction. Under the condition tested, BDE-47 did not inhibit the growth of S. oneidensis MR-1. Water soluble Fe(III)-citrate and the solid minerals ferrihydrite [Fe(III)₂O₃•0.5H₂O] and goethite [Fe(III)OOH] were tested in this study. Under anoxic condition, the amounts of Fe(II) produced by S. oneidensis MR-1 varied among the Fe(III)s tested, which decreased in the order of Fe(III)-citrate > ferrihydrite > goethite. Under subsequent oxic condition, H₂O₂ was produced via O₂ reduction by S. oneidensis MR-1. The amounts of H₂O₂ detected also varied, which decreased in the order of the reactions with Fe(III)-citrate > goethite > ferrihydrite. S. oneidensis MR-1 maintained its ability to produce Fe(II) and H₂O₂ for up to seven anoxic/oxic cycles. At each end of anoxic/oxic cycle, HO● was detected. The amount of HO● produced decreased in the order of the reactions with ferrihydrite > goethite > Fe(III)-citrate, which was opposite to that of H₂O₂ detected. Compared to the controls without HO●, the amounts of BDE-47 in the reactions with HO● decreased. The more HO● in the reaction, the less amount of BDE-47 detected. Furthermore, no BDE-47 degradation was observed when HO● was scavenged or ferrihydrite was either omitted or replaced by nitrate. Finally, identification of degradation products, such as hydroxylated BDE-47 and trisBDE, dibromophenol and monobromophenol, suggested that OH-addition and Br-substitution by HO● were the main mechanisms for degrading BDE-47. Collectively, all these results demonstrated for the first time that the Fenton reaction driven by S. oneidensis MR-1 degraded BDE-47 effectively.

The effective mineral absorption and bioreduction were considered as two preferred processes to alleviate the bioavailability and toxicity of toxic trace metals. In this study, the bioreduction of hexavalent chromium (Cr(VI)) on goethite (FeOOH) in the presence of Pseudomonas aeruginosa (P. aeruginosa) was investigated with different environmental factors, including carbon source concentrations, pH, temperature and initial Cr(VI) concentrations. The characterization of FeOOH–P. aeruginosa indicated that P. aeruginosa was surrounded by FeOOH, which could provide the essential iron for bacterial growth and reduce Cr(VI) to Cr(III). The optimal experimental conditions for Cr(VI) (initial concentration: 35 mg L⁻¹) absorption (∼46%) and bioreduction (∼54%) involved a temperature of 45 °C and pH of 5.5. Meanwhile, extracellular polymeric substances (EPS) secreted by P. aeruginosa and its functional groups played important roles in the reduction of Cr(VI). They could reduce Cr(VI) to Cr(III) and transform to Cr(OH)₃ or Feₓ-Cr₍₁₋ₓ₎(OH)₃ precipitation. These results of this study are of significant importance to better understand the environmental geochemical behavior of Cr(VI) with the interactions between soil minerals and microorganisms.

As a well-known toxic element, antimony occurred in a wide range of concentrations in the geothermal waters discharging from Rehai and Daggyai, two representative hydrothermal areas in the Yunnan-Sichuan-Tibet Geothermal Province of China. Antimony speciation in different types of the hot springs in Rehai and Daggyai varied greatly as well, and tri- and tetrathioantimonate were detected in most neutral to alkaline Rehai hot springs. Neutral to alkaline pH, high sulfide concentrations, and high sulfide to antimony ratios were the critical factors promoting the formation of thioantimonates. The fact that no thioantimonates were detected in neutral to alkaline Daggyai hot springs is attributed to high concentrations of coexistent arsenic capable of inhibiting the thiolation of oxyantimony anions, because thioantimonates are kinetically more labile than thioarsenates. Upon discharge of the hot springs, both total aqueous antimony and arsenic decreased rapidly and substantially via immobilization to the sediments in the spring vents and their outflow channels. Some of the common iron-bearing minerals in the spring sediments, like pyrite and goethite, are known sinks for antimony and arsenic. Yet, an interesting difference was observed with antimony and iron contents in the sediment samples showing a significant correlation that was lacking for arsenic and iron contents. The explanation might be that for arsenic, sorption affinities are known to vary significantly with aqueous arsenic speciation and mineral phases. Typically, thiolation increases, and oxidation decreases arsenic mobility. Sorption experiments for antimony conducted in the present study, in contrast, showed that different antimony species were comparably sorbed to pyrite over a wide range of initial antimony concentrations and to goethite at relatively low initial antimony concentrations (but still covering the concentration range of antimony in common natural waters), so neither thiolation nor oxidation contributed significantly to the mobility of antimony in the hot springs investigated in this study.

Bacteria with arsenate-reducing (ars) and arsenite-oxidizing (aio) genes usually co-exist in aerobic environments, but their contrast impacts on arsenic (As) speciation and mobility remain unclear. To identify which kind of bacteria dominate As speciation under oxic conditions, we studied the biotransformation of adsorbed As on goethite in the co-existence of Pantoea sp. IMH with ars gene and Achromobacter sp. SY8 with aio gene. The incubation results show that SY8 dominated the dissolved As speciation as As(V), even though aio exhibited nearly 5 folds lower transcription levels than ars in IMH. Nevertheless, our XANES results suggest that SY8 showed a negligible effect on solid-bound As speciation whereas IMH reduced adsorbed As(V) to As(III). The change in As speciation on goethite surfaces led to a partial As structural change from bidentate corner-sharing to monodentate corner-sharing as evidenced by our EXFAS analysis. Our Mössbauer spectroscopic results suggest that the incubation with SY8 reduced the degree of crystallinity of goethite, and the reduced crystallinity can be partly compensated by IMH. The changes in As adsorption structure and in goethite crystallinity had a negligible effect on As release. The insights gained from this study improve our understanding of biotransformation of As in aerobic environment.

Paddy soils around mining areas suffer from the great threat of heavy metal pollution. The traditional source-tracing methods based on metal concentrations limit our ability to quantify the sources of heavy metals and trace their transport processes to paddy soils. In this study, Zn isotope compositions of paddy soils in Dabaoshan mine area, a typical sulfide deposit in southern China, have been systematically studied. According to a plot between 1/Zn (i.e. inverse concentration) and δ⁶⁶Zn value, all the polluted paddy soils fall on the mixing line between acid mine drainage precipitate (AMD-precipitate) and fertilizer while the unpolluted paddy soil falls on the mixing line between fertilizer and bedrock. This indicates the mixing of Zn sources at least three end-members: the mining end-member (i.e. AMD-precipitate), the agricultural end-member (i.e. fertilizer), and bedrock whose geochemical signature is often overprinted by the former two sources around the mining area. The quantitative calculations to apportion the end-member’s contributions show that the mining activity contributes most Zn in the paddy soils with an average of ∼66.2%. The contribution of mining activities has significant spatial variations. Specifically, the mining activities have relatively low impacts on the lower reach and the deep soil. Additionally, the apparent Zn isotope fractionation between AMD and AMD-precipitate (Δ⁶⁶ZnAMD₋ₚᵣₑcᵢₚᵢₜₐₜₑ − AMD of −0.35 to −0.08‰) in the tailings dam suggests that Zn cations in AMD coprecipitated with the secondary Fe-bearing minerals (e.g. jarosite and goethite). After being discharged from the tailings dam, Zn is mainly carried by the Fe-oxide minerals and migrated during surface runoff. Our study highlights the contribution of human activities to the Zn pollution in the paddy soils and the key role of Fe-bearing minerals in the migration of Zn. These findings provide a scientific base for the development of policy for pollution control in mining-affected region.

Goethite is a common iron hydroxide, which can be substituted by manganese (Mn) in the goethite structure. It is important to investigate the immobilization of uranium(VI) on Mn-substituted goethite (Mn-Goe) to understand the fate and migration of uranium in soils and sediments. In this study, the sorption of uranium(VI) by Mn-Goe was investigated as a function of pH, adsorbent dosage, contact time, and initial uranium concentration in batch experiments. Several material analysis techniques were used to characterize manganese substituted materials. Results indicated that Mn was successfully introduced into the goethite structure, the length of particles increased gradually, the surface clearly exhibited higher roughness with increasing Mn content, and that uranium(VI) sorption of synthetic Mn-Goe appeared to be higher than that of goethite. The sorption kinetics supported the results presented by the pseudo-second-order model. The sorption capacity of uranium on Mn-Goe was circa 77 mg g⁻¹ at pH = 4.0 and 25 °C. Fourier transform-infrared spectroscopy (FT-IR) analyses revealed that uranium ions were adsorbed through functional groups containing oxygen on the Mn-Goe structure. The enhancement of Mn-substitution for the uranium(VI) sorption capacity of goethite was revealed. This study suggests that goethite and Mn-Goe can both play a significant role in controlling the mobility and transport of uranium(VI) in the subsurface environment, which is helpful for material development in environmental remediation.

Heavy metal pollution is very common in soils. Soils are complex systems including minerals, bacteria, and various other substances. In Cd(II) contaminated soil, the combined effects of clay minerals and heavy metals on bacterial biofilm and Cd(II) adsorption are unappreciated. Our study showed that the combination of clay minerals (goethite, kaolinite, and montmorillonite) and heavy metals promoted Serratia marcescens S14 biofilm development significantly more than clay minerals or Cd(II) alone. The amount of biofilm after binary treatment with clay minerals and Cd(II) was 2.3–7.3 times than that in control. Mineral-induced cell death and the expression of the fimA, bsmA, and eps were key players in biofilm formation. Binary treatment with montmorillonite and Cd(II) significantly enhanced biofilm development and consequently increased the adsorption of Cd(II). Cd(II) removal is the result of co-adsorption of bacteria and minerals. Bacterial biofilm played an important role in Cd(II) adsorption. FTIR spectroscopy showed the components of biofilm were not affected by minerals and revealed the functional groups –OH, –NH, –CH₂, –SH, –COO participated in Cd(II) immobilization. Our findings are of fundamental significance for understanding how minerals and Cd(II) affect biofilms and thereby enhance Cd(II) adsorption and predicting the mobility and fate of heavy metals in heavy metal-contaminated soil.

The biodegradable S,S-ethylenediaminedisuccinic acid (EDDS) is a promising chelant for chelant-assisted phytoextraction of trace metals in polluted soil. The interactions between EDDS and trace metals/major elements in the soil affect the metal bioavailability and their subsequent phytoextraction efficiency. This study aimed to investigate the macroscopic and molecular-level interactions of EDDS with Cu in the rhizosphere and non-rhizosphere of a Cu-polluted agricultural soil. A multi-interlayer rhizobox planted with ryegrass was used to simulate the transport of EDDS and Cu from the non-rhizosphere to rhizosphere soils. The results showed that EDDS (5 mM kg⁻¹) significantly dissociated Cu (285–690 fold), Fe (by 3.47–60.2 fold), and Al (2.43–5.31 fold) from the soil in comparison with a control group. A combination of micro-X-ray fluorescence, X-ray absorption near-edge structure spectroscopy, and sequential extraction analysis revealed that EDDS primarily chelated the adsorbed fraction of Cu by facilitating the dissolution of goethite. Moreover, as facilitated by ryegrass transpiration, CuEDDS was moved from the non-rhizosphere to rhizosphere and accumulated in ryegrass. In situ processes of Cu extraction and transport by EDDS in the rhizosphere were further elucidated with chemical speciation analysis and geochemical modeling methods.

The pollution of continental shelf sea sediments has always been an important issue for scientists because it can directly affect marine life and marine ecology. Thus, we carried out detailed studies of environmental magnetism and heavy metals in 145 surface sediment samples from the area southeast of the Zhoushan Islands in the East China Sea. The magnetic minerals in the sediments are mainly magnetite with a small amount of goethite and hematite. The magnetic mineral particles are mainly pseudosingle domain and contain a certain amount of superparamagnetic particles. The distribution of the magnetic mineral content shows a gradual decreasing trend from land to sea, which is closely related to the sediment transport pattern. The anhysteretic remanent magnetization (χARM) and χARM ratio to magnetic susceptibilities (χARM/χ) were more sensitive than other indexes to sediment pollution and could be used as indicators for environmental pollution in the nearshore area. Additionally, a correlation analysis with the major elements showed that the χARM ratio to saturation isothermal remanence (χARM/SIRM) can well explain the sources of terrigenous sediment. Therefore, the environmental magnetic parameters in the continental shelf area can be used as a precursor for geochemical research and provide data support for further research.