International audience | Aided phytostabilization using a combination of compost, zerovalent iron grit and coal fly ash (CZA) amendments and revegetation effectively promoted the biological recovery of mining spoils generated at a gold mine in Portugal. Selective dissolution of spoil samples in combination with solid phase characterization using microbeam X-ray absorption near edge structure (μXANES) spectroscopy and microbeam X-ray fluorescence (μXRF) mapping were used to assess As associations in spoils ten years after CZA treatment. The results show that As preferentially associates with poorly crystalline Fe-oxyhydroxides as opposed to crystalline Fe-(oxyhydr)oxide phases. The crystalline Fe(III)-phases dominated in the treated spoil and exceeded those of the untreated spoil three-fold, but only 2.6–6.8% of total As was associated with this fraction. Correlation maps of As:Fe reveal that As in the CZA-treated spoils is primarily contained in surface coatings as precipitates and sorbates. Arsenic binding with poorly crystalline Fe-oxyhydroxides did not inhibit As uptake by plants.

Physicochemical and biological characterisation of different dredged sediment deposit sites in France

Inhalation of respirable silica particles can cause serious lung diseases (e.g., silicosis and lung cancer), and the toxicity of respirable silica is highly dependent on its crystal form. Common combustion processes such as coal and biomass burning can provide high temperature environments that may alter the crystal forms of silica and thus affect its toxic effects. Although crystalline silica (i.e., quartz, tridymite, and cristobalite) were widely found at different temperatures during the burning processes, the sources and crystal transformation pathways of silica in the burning processes are still not well understood. Here, we investigate the crystal transformation of silica in the coal and biomass combustion processes and clarify the detailed transformation pathways of silica for the first time. Specifically, in coal burning process, amorphous silica can transform into quartz and cristobalite starting at 1100 °C, and quartz transforms into cristobalite starting at 1200 °C; in biomass burning process, amorphous silica can transform into cristobalite starting at 800 °C, and cristobalite transforms into tridymite starting at 1000 °C. These transformation temperatures are significantly lower than those predicted by the classic theory due to possibly the catalysis of coexisting metal elements (e.g., aluminum, iron, and potassium). Our results not only enable a deeper understanding on the combustion-induced crystal transformation of silica, but also contribute to the mitigation of population exposure to respirable silica.

The effectiveness and feasibility of the three biochar materials for remediation of arsenic (As) and lead (Pb) contaminated soil were explored in this study. Significant reduction of bioaccessibility and migration risks of both heavy metals have been explained mechanistically by incubation, column experiments and numerical simulation. Langmuir equation fitted As and Pb sorption isotherms better in the control and biochar (BC) amended soils, while Freundlich model was more suitable for iron modified biochar (Fe-BC) and sulfur/iron modified biochar (S/Fe-BC) amended soils, indicating that modified biochar promoted chemical adsorption process for As and Pb. For the three biochar materials, S/Fe-BC showed the best effects on reducing the bioavailability of As and Pb, with a decrease of 40.42%–64.21%. The reduction in bioaccessibility by metal portioning into available and non-available fractions was better for illustrating the mechanisms including adsorption, precipitation/coprecipitation and As(III) oxidation behind S/Fe-BC efficacy. Moreover, S/Fe-BC can effectively inhibit the leaching behavior of As and Pb under acid rain, which increased by 99.89% and 90.18%, respectively, compared with the control. The HYDRUS-1D modeling indicated that S/Fe-BC could continuously treat As (100 mg/L) and Pb (1000 mg/L) contaminated water for 16.22 years and 40.86 years, respectively, and ensure the groundwater quality criteria being met. Based on these insights, we believe that our study will provide meaningful information about the potentials of biochar derived materials for soil heavy metals’ remediation.

Metal oxide-modified biochar showed excellent adsorption performance in wastewater treatment. Iron nitrate and potassium permanganate were oxidative modifiers through which oxygen-containing groups and iron–manganese oxides could be introduced into biochar. In this study, iron–manganese (Fe–Mn) oxide-modified biochar (BC-FM) was synthesized using rice straw biochar, and the adsorption process, removal effect, and the mechanism of cadmium (Cd) adsorption on BC-FM in wastewater treatment were explored through batch adsorption experiments and characterization (SEM, BET, FTIR, XRD, and XPS). Adsorption kinetics showed that the maximum adsorption capacity of BC-FM for Cd(II) was 120.77 mg/g at 298 K, which was approximately 1.5–10 times the amount of adsorption capacity for Cd(II) by potassium-modified or manganese-modified biochar as mentioned in the literature. The Cd(II) adsorption of BC-FM was well fit by the pseudo-second-order adsorption and Langmuir models, and it was a spontaneous and endothermic process. Adsorption was mainly controlled via a chemical adsorption mechanism. Moreover, BC-FM could maintain a Cd removal rate of approximately 50% even when reused three times. Cd(II) capture by BC-FM was facilitated by coprecipitation, surface complexation, electrostatic attraction, and cation-π interaction. Additionally, the loaded Fe–Mn oxides also played an important role in the removal of Cd(II) by redox reaction and ion exchange in BC-FM. The results suggested that BC-FM could be used as an efficient adsorbent for treating Cd-contaminated wastewater.

Geochemical cycling of iron (Fe) mediated by sediment microbes drives the remobilization of phosphorus (P). Understanding the underlying mechanism is essential for the evaluation of P retention by wetlands. The diffusive gradients in thin film (DGT) and 16S rDNA sequencing techniques were combined to explore seasonal variations in the remobilization mechanism of sediment P in a free water surface wetland in southwest China. A significantly positive correlation between labile P and Fe concentrations was found from the sediment profiles, indicating coupled remobilization of Fe and P in the sediment. Fe-reducing bacterial genera, particularly Sphingomonas and Geothermobacter, were responsible for the reductive dissolution of Fe oxides and subsequent P release in sediment. The efflux of sediment P was higher in the rainy season (95 ± 87 ng cm⁻² d⁻¹) than in the dry season (39 ± 29 ng cm⁻² d⁻¹). Based on the significantly positive relationship between the efflux and total concentration of sediment P, we propose a promising regression equation for quantifying the release risk of sediment P. The Luoshijiang Wetland exhibited a higher release potential as indicated by a greater regression slope (0.558) compared to the other water bodies (0.055), which was mainly attributed to the lower labile Fe:P molar ratio in the sediment. Based on estimations of the diffusive flux of P at the sediment-water interface, sediment contributed more than 172 and 413 g of P per day to the water column in the dry and rainy seasons, respectively, accounting for 14.0% and 1.9% of the P mass in the surface water of the wetland.

Modification of biochar by low-cost iron sources has gained increasing attention to improve pollutants removal performance and reduce production costs compared to conventional chemical modifications. While such iron sources generally have complex compositions, their effects on properties of the iron-biochar composite are not well investigated. This study produced an iron-biochar (RBC) composite from co-pyrolysis of incinerated sewage sludge ash (ISSA) and peanut shell, and examined the role of silica with widespread existence in ISSA and other low-cost iron sources on properties of the iron-biochar composite relevant to As(III)/As(V) removal. Silica was found to react with iron during the pyrolysis process at 850 °C and formed iron silicon at the expense of producing zero valent iron and Fe₃O₄ which resulted in a poorer removal efficacy for As(III) and As(V) compared to the iron-biochar (FBC) made from pure Fe₂O₃ and peanut shell. Moreover, a high leaching of reactive silica from RBC was observed which affected the formation of corrosion products of ZVI and competed with arsenic for active adsorption sites. Despite this, RBC still exhibited a maximum adsorption capacity of 17.44 and 57.56 mg/g towards As(III) and As(V) respectively at pH 3.0. Overall, this study provides an interesting insight into upcycling ISSA into useful media for sorptive removal of arsenic from aqueous solutions.

Since lead, cadmium and arsenic have completely opposite chemical behaviors, it is very difficult to stabilize all these three heavy metals simultaneously. Herein, a novel iron-doped hydroxyapatite composite (Fe-HAP) was developed via an ultrasonic-assisted microwave hydrothermal method for the simultaneous remediation of lead-, cadmium-, and arsenic-co-contaminated soil in Hunan Province, South China. Using DTPA/sodium bicarbonate extractant to extract bioavailable Pb, Cd and As in soil after Fe-HAP remediation for 60 days, the immobilization efficiencies were 79.77%, 51.3% and 37.5% for Pb, Cd and As, respectively. The soil extractable and exchangeable fractions of Pb, Cd and As decreased significantly. In batch experiments, the adsorption kinetics of Pb, Cd and As on Fe-HAP were well described by pseudo-second-order models, indicating that the adsorption is controlled by chemisorption. In the Langmuir adsorption isotherm, the maximum adsorption capacities of Cd²⁺ and As(V) were 476.2 mg g⁻¹ and 195.69 mg g⁻¹, respectively, while Pb²⁺ fit the Freundlich model better. The XRD, SEM and XPS analyses indicated that Fe-HAP formed stable minerals of Pb₅(PO₄)₃OH, Cd₃(PO₄)₂·4H₂O, Cd(OH)₂ and Fe₃(AsO₄)₂·6H₂O with Pb, Cd and As. Overall, its facile and efficient immobilization performance indicate that Fe-HAP has potential for practical applications in integrative remediation of Pb-, Cd-, and As- co-contaminated soil.

Understanding the arsenic (As) enrichment mechanisms in the subsurface environment relies on a systematic investigation of As valence species and their partitioning with the Fe (oxyhydr)oxide phases in the subsoil profile. The present study explored the distribution, speciation, partitioning, and (im)mobilization of As associated with Fe in four subsoil cores (∼30 m depth) from Hong Kong using sequential chemical extraction and X-ray absorption near edge spectroscopy. The subsoil profiles exhibited relatively high concentrations of As at 26.1–982 mg/kg (median of 112 mg/kg), and the As was dominated by As(V) (85–96%) and primarily associated with the residual fraction (50.7–94.7%). A small amount of As (0.002–13.2 mg/kg) was easily mobilized from the four subsoil profiles, and a concentration of water-soluble As higher than 100 μg/L was observed for only some subsoil layers. The molar ratios of As:Fe in the oxalate-extractable Fe fraction ranged from 1.2 to 76.5 mmol/mol (median of 11.1 mmol/mol), revealing the participation of poorly crystalline Fe (oxyhydr)oxides in immobilizing most of the high geogenic As. The primary phases of ferric (oxyhydr)oxides were characterized as ferrihydrite (16–53%), lepidocrocite (0–32%), and goethite (0–62%), and these phases contributed to the sufficient ability of the subsoil to sequester 45.3–100% (median of 98.8%) of the exogenous As(V) (1.0 mg/L) in adsorption experiments. In contrast to As(V), exogenous As(III) showed a lower removal percentage (3.9–79.1%, median of 45.1%). The study revealed that the chemical speciation of As and Fe in the subsoil profiles is useful for predicting the immobilization of high geogenic As in the region, which is also helpful for the safe utilization of As-containing soil during land development worldwide.