Environmental Risk of Arsenic Mobilization from Disposed Sand Filter Materials.

Arsenic (As)-bearing water treatment residuals (WTRs) from household sand filters are usually disposed on top of floodplain soils and may act as a secondary As contamination source. We hypothesized that open disposal of these filter-sands to soils will facilitate As release under reducing conditions. To quantify the mobilization risk of As, we incubated the filter-sand, the soil, and a mixture of the filter-sand and soil in anoxic artificial rainwater and followed the dynamics of reactive Fe and As in aqueous, solid, and colloidal phases. Microbially mediated Fe(III)/As(V) reduction led to the mobilization of 0.1-4% of the total As into solution with the highest As released from the mixture microcosms equaling 210 µg/L. Due to the filter-sand and soil interaction, Mössbauer and X-ray absorption spectroscopies indicated that up to 10% Fe(III) and 32% As(V) were reduced in the mixture microcosm. Additionally, the mass concentrations of colloidal Fe and As analyzed by single-particle ICP-MS decreased by 77-100% compared to the onset of reducing conditions with the highest decrease observed in the mixture setups (>95%). Overall, our study suggests that (i) soil provides bioavailable components (e.g., organic matter) that promote As mobilization via microbial reduction of As-bearing Fe(III) (oxyhydr)oxides and (ii) As mobilization as colloids is important especially right after the onset of reducing conditions but its importance decreases over time.

Effect of Natural Organic Matter on the Fate of Cadmium During Microbial Ferrihydrite Reduction.

Natural organic matter (NOM) is known to affect the microbial reduction and transformation of ferrihydrite, but its implication toward cadmium (Cd) associated with ferrihydrite is not well-known. Here, we investigated how Cd is redistributed when ferrihydrite undergoes microbial reduction in the presence of NOM. Incubation with Geobacter sulfurreducens showed that both the rate and the extent of reduction of Cd-loaded ferrihydrite were enhanced by increasing concentrations of NOM (i.e., C/Fe ratio). Without NOM, only 3-4% of Fe(III) was reduced, but around 61% of preadsorbed Cd was released into solution due to ferrihydrite transformation to lepidocrocite. At high C/Fe ratio (1.6), more than 35% of Fe(III) was reduced, as NOM can facilitate bioreduction by working as an electron shuttle and decreased aggregate size, but only a negligible amount of Cd was released into solution, thus decreasing Cd toxicity and prolonging microbial Fe(III) reduction. No ferrihydrite transformation was observed at high C/Fe ratios using Mössbauer spectroscopy and X-ray diffraction, and X-ray absorption spectroscopy indicated the proportion of Cd-OM bond increased after microbial reduction. This study shows that the presence of NOM leads to less mobilization of Cd under reducing condition possibly by inhibiting ferrihydrite transformation and recapturing Cd through Cd-OM bond.

Arsenic(V) Incorporation in Vivianite during Microbial Reduction of Arsenic(V)-Bearing Biogenic Fe(III) (Oxyhydr)oxides.

The dissolution of arsenic-bearing iron(III) (oxyhydr)oxides during combined microbial iron(III) and arsenate(V) reduction is thought to be the main mechanism responsible for arsenic mobilization in reducing environments. Besides its mobilization during bioreduction, arsenic is often resequestered by newly forming secondary iron(II)-bearing mineral phases. In phosphate-bearing environments, iron(II) inputs generally lead to vivianite precipitation. In fact, in a previous study we observed that during bioreduction of arsenate(V)-bearing biogenic iron(III) (oxyhydr)oxides in phosphate-containing growth media, arsenate(V) was immobilized by the newly forming secondary iron(II) and iron(II)/iron(III)mineral phases, including vivianite. In the present study, changes in arsenic redox state and binding environment in these experiments were analyzed. We found that arsenate(V) partly replaced phosphate in vivianite, thus forming a vivianite-symplesite solid solution identified as Fe3(PO4)1.7(AsO4)0.3·8H2O. Our data suggests that in order to predict the fate of arsenic during the bioreduction of abiogenic and biogenic iron(III) (oxyhydr)oxides in arsenic-contaminated environments, the formation of symplesite-vivianite minerals needs to be considered. Indeed, such mineral phases could contribute to a delayed and slow release of arsenic in phosphate-bearing surface and groundwater environments.

Rhizosphere microbial community composition affects cadmium and zinc uptake by the metal-hyperaccumulating plant Arabidopsis halleri.

The remediation of metal-contaminated soils by phytoextraction depends on plant growth and plant metal accessibility. Soil microorganisms can affect the accumulation of metals by plants either by directly or indirectly stimulating plant growth and activity or by (im)mobilizing and/or complexing metals. Understanding the intricate interplay of metal-accumulating plants with their rhizosphere microbiome is an important step toward the application and optimization of phytoremediation. We compared the effects of a "native" and a strongly disturbed (gamma-irradiated) soil microbial communities on cadmium and zinc accumulation by the plant Arabidopsis halleri in soil microcosm experiments. A. halleri accumulated 100% more cadmium and 15% more zinc when grown on the untreated than on the gamma-irradiated soil. Gamma irradiation affected neither plant growth nor the 1 M HCl-extractable metal content of the soil. However, it strongly altered the soil microbial community composition and overall cell numbers. Pyrosequencing of 16S rRNA gene amplicons of DNA extracted from rhizosphere samples of A. halleri identified microbial taxa (Lysobacter, Streptomyces, Agromyces, Nitrospira, "Candidatus Chloracidobacterium") of higher relative sequence abundance in the rhizospheres of A. halleri plants grown on untreated than on gamma-irradiated soil, leading to hypotheses on their potential effect on plant metal uptake. However, further experimental evidence is required, and wherefore we discuss different mechanisms of interaction of A. halleri with its rhizosphere microbiome that might have directly or indirectly affected plant metal accumulation. Deciphering the complex interactions between A. halleri and individual microbial taxa will help to further develop soil metal phytoextraction as an efficient and sustainable remediation strategy.

Are rice (Oryza sativa L.) phosphate transporters regulated similarly by phosphate and arsenate? A comprehensive study.

Rice is one of the most important staple foods worldwide, but it often contains inorganic arsenic, which is toxic and gives rise to severe health problems. Rice plants take up arsenate As(V) via the phosphate transport pathways, though it is not known how As(V), as compared to phosphate, modifies the expression of phosphate transporters (PTs). Therefore, the impact of As(V) or phosphate (Pi) on the gene expression of PTs and several Pi signaling regulators was investigated. Rice plants were grown on medium containing different As(V) or Pi concentrations. Growth was evaluated and the expression of tested genes was quantified at different time points, using quantitative RT-PCR (qPCR). The As and P content in plants was determined using inductively coupled plasma mass spectrometry (ICP-MS). As(V) elicited diverse and opposite responses of different PTs in roots and shoots, while Pi triggered a more shallow and uniform transcriptional response in several tested genes. Only a restricted set of genes, including PT2, PT3, PT5 and PT13 and two SPX-MFS family members, was particularly responsive to As(V). Despite some common reactions, the responses of the analyzed genes were predominantly ion-specific. The possible reasons and consequences are discussed.

Granulation compared to co-application of biochar plus mineral fertilizer and its impacts on crop growth and nutrient leaching.

Mechanized biochar field application remains challenging due to biochar's poor flowability and bulk density. Granulation of biochar with fertilizer provides a product ready for application with well-established machinery. However, it's unknown whether granulated biochar-based fertilizers (gBBF) are as effective as co-application of non-granulated biochar with fertilizer. Here, we compared a gBBF with a mineral compound fertilizer (control), and with a non-granulated biochar that was co-applied at a rate of 1.1 t ha-1 with the fertilizer in a white cabbage greenhouse pot trial. Half the pots received heavy rain simulation treatments to investigate nutrient leaching. Crop yields were not significantly increased by biochar without leaching compared to the control. With leaching, cabbage yield increased with gBBF and biochar-co-application by 14% (p > 0.05) and 34% (p < 0.05), respectively. Nitrogen leaching was reduced by 26-35% with both biochar amendments. Biochar significantly reduced potassium, magnesium, and sulfur leaching. Most nitrogen associated with gBBF was released during the trial and the granulated biochar regained its microporosity. Enriching fertilizers with biochar by granulation or co-application can improve crop yields and decrease nutrient leaching. While the gBBF yielded less biomass compared to biochar co-application, improved mechanized field application after granulation could facilitate the implementation of biochar application in agriculture.

Fate of arsenic during microbial reduction of biogenic versus Abiogenic As-Fe(III)-mineral coprecipitates.

Fe(III) (oxyhydr)oxide minerals exhibit a high sorption affinity for arsenic (As) and the reductive dissolution of As-bearing Fe(III) (oxyhydr)oxides is considered to be the primary mechanism for As release into groundwater. To date, research has focused on the reactivity of abiogenic Fe(III) (oxyhydr)oxides, yet in nature biogenic Fe(III) (oxyhydr)oxides, precipitated by Fe(II)-oxidizing bacteria are also present. These biominerals contain cell-derived organic matter (CDOM), leading to different properties than their abiogenic counterparts. Here, we follow Fe mineralogy and As mobility during the reduction of As-loaded biogenic and abiogenic Fe(III) minerals by Shewanella oneidensis MR-1. We found that microbial reduction of As(III)-bearing biogenic Fe(III) (oxyhydr)oxides released more As than reduction of abiogenic Fe(III) (oxyhydr)oxides. In contrast, As was immobilized more effectively during reduction of As(V)-loaded biogenic than abiogenic Fe(III) (oxyhydr)oxides during secondary Fe mineral formation. During sterile incubation of minerals and after microbial Fe(III) reduction stopped, As(V) was mobilized from biogenic Fe(III) (oxyhydr)oxides probably by sorption competition with phosphate and CDOM. Our data show that the presence of CDOM significantly influences As mobility during reduction of Fe(III) minerals and we suggest that it is essential to consider both biogenic and abiogenic Fe(III) (oxyhydr)oxides to further understand the environmental fate of As.

Organic carbon and reducing conditions lead to cadmium immobilization by secondary Fe mineral formation in a pH-neutral soil.

Cadmium (Cd) is of environmental relevance as it enters soils via Cd-containing phosphate fertilizers and endangers human health when taken up by crops. Cd is known to associate with Fe(III) (oxyhydr)oxides in pH-neutral to slightly acidic soils, though it is not well understood how the interrelation of Fe and Cd changes under Fe(III)-reducing conditions. Therefore, we investigated how the mobility of Cd changes when a Cd-bearing soil is faced with organic carbon input and reducing conditions. Using fatty acid profiles and quantitative PCR, we found that both fermenting and Fe(III)-reducing bacteria were stimulated by organic carbon-rich conditions, leading to significant Fe(III) reduction. The reduction of Fe(III) minerals was accompanied by increasing soil pH, increasing dissolved inorganic carbon, and decreasing Cd mobility. SEM-EDX mapping of soil particles showed that a minor fraction of Cd was transferred to Ca- and S-bearing minerals, probably carbonates and sulfides. Most of the Cd, however, correlated with a secondary iron mineral phase that was formed during microbial Fe(III) mineral reduction and contained mostly Fe, suggesting an iron oxide mineral such as magnetite (Fe3O4). Our data thus provide evidence that secondary Fe(II) and Fe(II)/Fe(III) mixed minerals could be a sink for Cd in soils under reducing conditions, thus decreasing the mobility of Cd in the soil.

Fate of Cd during microbial Fe(III) mineral reduction by a novel and Cd-tolerant Geobacter species.

Fe(III) (oxyhydr)oxides affect the mobility of contaminants in the environment by providing reactive surfaces for sorption. This includes the toxic metal cadmium (Cd), which prevails in agricultural soils and is taken up by crops. Fe(III)-reducing bacteria can mobilize such contaminants by Fe(III) mineral dissolution or immobilize them by sorption to or coprecipitation with secondary Fe minerals. To date, not much is known about the fate of Fe(III) mineral-associated Cd during microbial Fe(III) reduction. Here, we describe the isolation of a new Geobacter sp. strain Cd1 from a Cd-contaminated field site, where the strain accounts for 10(4) cells g(-1) dry soil. Strain Cd1 reduces the poorly crystalline Fe(III) oxyhydroxide ferrihydrite in the presence of at least up to 112 mg Cd L(-1). During initial microbial reduction of Cd-loaded ferrihydrite, sorbed Cd was mobilized. However, during continuous microbial Fe(III) reduction, Cd was immobilized by sorption to and/or coprecipitation within newly formed secondary minerals that contained Ca, Fe, and carbonate, implying the formation of an otavite-siderite-calcite (CdCO3-FeCO3-CaCO3) mixed mineral phase. Our data shows that microbially mediated turnover of Fe minerals affects the mobility of Cd in soils, potentially altering the dynamics of Cd uptake into food or phyto-remediating plants.

Rice production threatened by coupled stresses of climate and soil arsenic.

Projections of global rice yields account for climate change. They do not, however, consider the coupled stresses of impending climate change and arsenic in paddy soils. Here, we show in a greenhouse study that future conditions cause a greater proportion of pore-water arsenite, the more toxic form of arsenic, in the rhizosphere of Californian Oryza sativa L. variety M206, grown on Californian paddy soil. As a result, grain yields decrease by 39% compared to yields at today's arsenic soil concentrations. In addition, future climatic conditions cause a nearly twofold increase of grain inorganic arsenic concentrations. Our findings indicate that climate-induced changes in soil arsenic behaviour and plant response will lead to currently unforeseen losses in rice grain productivity and quality. Pursuing rice varieties and crop management practices that alleviate the coupled stresses of soil arsenic and change in climatic factors are needed to overcome the currently impending food crisis.

Heavy metal mobility and valuable contents of processed municipal solid waste incineration residues from Southwestern Germany.

As conventional end-of-life disposal, municipal solid waste (MSW) incineration residues can be problematic due to potential release of toxic compounds into the environment. Using municipal solid waste incineration residues as urban-mine of valuable metals (e.g. precious metals) could provide a trash-to-treasure possibility. The objectives of the study are to (i) determine the contents of different contaminant metallic elements (Zn, Cu, Ba, Pb, Cr and Ni) in four size fractions of MSW incineration residues and discuss their mobility potential by using the modified BCR sequential extraction method; (ii) investigate the level of valuable critical contents (precious metals, rare earth elements, etc.) in these wastes. We also characterized mineralogy and elemental composition of four different grain size fractions (0-0.5, 0.5-2.0, 2.0-4.0 and 4.0-16.0 mm) of processed municipal solid waste incineration residue (PIR) from the Southwestern region of Germany, using X-ray fluorescence, X-ray powder diffraction and different spectroscopic techniques. Among all studied size fractions, grains smaller than 2 mm contained higher amounts of total extractable heavy metals in most cases. The most important finding of the study is that the total contents of Cu, Au and Pt in the incineration residues reached economically profitable levels (5.1 g/kg, 21.69 mg/kg and 17.45 mg/kg, respectively).