Copper Mobilization and Immobilization along an Organic Matter and Redox Gradient-Insights from a Mofette Site

Polystyrene influence on Pb bioavailability and rhizosphere toxicity: Challenges for ramie (Boehmeria nivea L.) in soil phytoremediation

Microplastics (MPs) and heavy metals often coexist in soil, however their interactions and effects on the soil-plant system remain largely unclear. In this study, ramie (Boehmeria nivea L.) was exposed to soil contaminated with lead (Pb) and polystyrene (PS) of different sizes, dosages, and surface-charged functional groups. This design aimed to simulate the effects of MPs on phytoremediation. The experimental results revealed that PS exacerbated the damaging effects of Pb on ramie. Compared to the effect of Pb alone, PS-COOH had a greater influence on root vigor, leading to a 15.6 % reduction in the active absorption ratio. Laser scanning confocal microscope showed PS entered the roots. Adsorption/desorption experiments demonstrated that PS had a weaker adsorption capacity for Pb than soil but a greater desorption rate than soil when simulating rhizosphere secretion. Moreover, PS reduced soil pH and increased the reducible state of Pb by 6-12 %. After 100 days of phytoremediation, Pb content in the soil with PS-5 μm was 150 μg g-1 less than that in the soil without PS. These results demonstrated that PS improved Pb bioavailability and enhanced the efficiency of Pb uptake by ramie. The redundancy analysis demonstrated that PS mitigated the toxicity of Pb to rhizosphere microorganisms, potentially via its effects on metal chemical fractions, dehydrogenase activity (S-DHA), cation exchange capacity (CEC), and soil organic matter (SOM). This study indicates that the presence of PS could potentially enhance the phytoremediation efficiency of ramie in Pb-contaminated land by influencing soil microenvironmental properties. This study provides insights into the complex interactions of MPs with soil-plant-microbial systems during metal remediation, thereby enhancing our understanding of their environmental impacts.

Soil moisture dynamics regulates the release rates and lability of copper in contaminated paddy soils

The climate changes have caused more extreme precipitation and drought events in the field and have exacerbated the severity of wet-dry events in soils, which will inevitably lead to severe fluctuations in soil moisture content. Soil moisture content has been recognized to influence the distribution of heavy metals, but how temporal changes of soil moisture dynamics affect the release rates and lability of heavy metals is still poorly understood, which precludes accurate prediction of environmental behavior and environmental risk of heavy metals in the field. In this study, we combined experimental and modeling approaches to quantify copper release rates and labile copper fractions in two paddy soils from southern China under different moisture conditions. Our kinetic data and models showed that the release rates and lability of copper were highly associated with the soil moisture contents, in which, surprisingly, high soil moisture contents effectively reduced the release rates of copper even with little changes in the reactive portions of copper in soils. A suite of comprehensive characterization on soil solid and solution components along the incubation suggested that soil microbes may regulate soil copper lability through forming microbially derived organic matter that sequestered copper and by increasing soil particle aggregation for protecting copper from release. This study highlights the importance of incorporating soil moisture dynamics into future environmental models. The experimental and modeling approaches in this study have provided basis for further developing predictive models applicable to paddy soils with varying soil moisture under the impact of climate change.

Multi-metal contaminant mobilizations by natural colloids and nanoparticles in paddy soils during reduction and reoxidation

Naturally-occurring colloids and nanoparticles are crucial in transporting heavy metal contaminants in soil-water systems. However, information on particle-bound metals' size distribution and elemental composition in paddy soils under redox-fluctuation is scarce. Here, we investigated the mobilization of Cu, Cd, and Pb-containing nanoparticles and colloids in four contaminated soils with distinctive geochemical properties during reduction and subsequent re-oxidation. Using AF4-UV-ICP-MS and STEM-EDS, we observed that particle-bound metals were primarily associated with two sizes ranges: 0.3-40 kDa (F1) and 130 kDa-450 nm (F2), which mainly consisted of organic matter (OM), iron hydroxide and clay minerals. Cu and Pb were more likely bound to colloid than Cd. Colloidal Cu, Pb and Cd accounted for averages of 83.2%, 72.4% and 19.8% of their total concentration in solution (<0.45 µm) during soil reduction, and decreased during soil re-oxidation. This proportion was also positively correlated with aqueous pH and DOC but negatively correlated with Eh. Further quantitative analysis demonstrated that Cu/Cd positively correlated with OM at nanometric scale (F1). This study provides quantitative insights into the size, composition and abundance of polymetallic pollutant-carrying particles in paddy soils during redox fluctuation, and highlights the importance of nanometric interactions between OM and toxic cationic metals for their release.

Copper bioreduction and nanoparticle synthesis by an enrichment culture from a former copper mine

Microorganisms can facilitate the reduction of Cu2+ , altering its speciation and mobility in environmental systems and producing Cu-based nanoparticles with useful catalytic properties. However, only a few model organisms have been studied in relation to Cu2+ bioreduction and little work has been carried out on microbes from Cu-contaminated environments. This study aimed to enrich for Cu-resistant microbes from a Cu-contaminated soil and explore their potential to facilitate Cu2+ reduction and biomineralisation from solution. We show that an enrichment grown in a Cu-amended medium, dominated by species closely related to Geothrix fermentans, Azospira restricta and Cellulomonas oligotrophica, can reduce Cu2+ with subsequent precipitation of Cu nanoparticles. Characterisation of the nanoparticles with (scanning) transmission electron microscopy, energy-dispersive x-ray spectroscopy and electron energy loss spectroscopy supports the presence of both metallic Cu(0) and S-rich Cu(I) nanoparticles. This study provides new insights into the diversity of microorganisms capable of facilitating copper reduction and highlights the potential for the formation of distinct nanoparticle phases resulting from bioreduction or biomineralisation reactions. The implications of these findings for the biogeochemical cycling of copper and the potential biotechnological synthesis of commercially useful copper nanoparticles are discussed.

Consequences of sea level rise for high metal(loid) loads in the Ría of Huelva estuary sediments

Ría of Huelva, located in southwestern Spain, is a highly metal(loid)-contaminated estuary system where sediments are exceeding action limits in an increasing order for Cd, Zn, Pb, Cu, and As. With a predicted sea level rise over the next 50 years, the estuary will be subject to flooding with brackish water or seawater. To evaluate the risk of metal(loid) mobilization under future climate scenarios, different locations along the estuary were sampled at different depths. Samples were flooded with river water, brackish water, and seawater under different short- and long-term laboratory setups. Potential metal(loid) mobilization showed that water quality standards for As, Pb, Zn, Ni, Cu, and Cd could be exceeded upon seawater flooding. However, metal(loid) mobilization was not predictable solely based on sediment loads. The driving factors for cation and anion mobility were identified to be mainly pH under low salinity and competitive desorption under high salinity conditions. Further drivers such as wave movement or labile C input in C-limited systems were found to enhance metal(loid) mobilization. Long-term flooding of intact sediment cores revealed that sea level rise will have different effects on the estuary system depending on duration of flooding. Short-term flooding in the near future will first affect alkaline sediments and enhance currently low cation mobilization, while anion mobilization due to reductive Fe dissolution will remain high. Once acidic sediments further inland are flooded with seawater, highest contaminant mobilization can be expected as high salinity will further enhance already high cation mobilization under acidic pH. Long-term flooding with seawater will neutralize the sediment pH and limit cation mobilization compared to acidic pH. However, the contaminant load stored in the estuary is so high that, extrapolating data obtained, mobilization could last for >1000 years, e.g. for As, Pb, and Al.

Mechanisms of S cooperating with Fe and Mn to regulate the conversion of Cd and Cu during soil redox process revealed by LDHs-DGT technology

The activity changes of Cd and Cu in paddy field were strongly influenced by the transformation of S, Fe and Mn species. However, in the process of soil redox, how S cooperates with Fe/Mn to regulate the law and mechanism of Cd and Cu speciation transformation still needs to be studied. In this study, we used DGT technology based on layer double hydroxides (LDHs) combined with pore water sampling to investigate soil redox changes, rice growth, and the effects of different forms of sulfur (S⁰, SO₄^2-) on soil Cd and Cu activities. The results showed that the concentrations of C_DGT-Cd and Cu in the soil decreased rapidly in the anaerobic stage, but increased slowly in the oxidative stage. Multiple regression analysis showed that the changes of Cu and Cd concentrations mainly depended on the changes of Fe/Mn morphology. Sulfur treatment promoted the dissolution of Fe/Mn oxides in the short term (<48 h), and the activities of C_DGT-Fe, Mn, and Cd increased simultaneously, but C_DGT-Cu was not affected. However, after long-term anaerobic conditions (>10 d), sulfur addition reduced the activities of C_DGT-Cd and Cu, and decreased the uptake of Cd and Cu by rice. During sulfate reduction, the sulfur addition treatment group resulted in a 24.5-50.2 % decrease in C_DGT-Fe, indicating that sulfur addition may delay the release of Cd and Cu after rice planting by promoting the formation of FeS/FeS₂. In addition, in the anaerobic stage, Cu formed sulfide before Cd and was fixed, and the higher thermodynamic stability of CuS would promote the dissolution of CdS in the oxidation stage. Overall, soil flooding with sulfur to enhance the generation of metal sulfides and secondary iron ores provides an opportunity to use sulfur as an environmentally friendly modifier to coordinate Fe, Mn to improve heavy metal-contaminated soils.

Trace Metal(loid) Migration from Road Dust to Local Vegetables and Tree Tissues and the Bioaccessibility-Based Health Risk: Impacts of Vehicle Operation-Associated Emissions

Traffic activities release large amounts of trace metal(loid)s in urban environments. However, the impact of vehicle operation-associated emissions on trace metal(loid) enrichment in road dust and the potential migration of these trace metal(loid)s to the surrounding environment remain unclear. We evaluated the contamination, sequential fraction, and bioaccessibility of trace metal(loid)s in urban environments by assessing their presence in road dust, garden vegetables, and tree tissues, including bark and aerial roots, at a traffic-training venue impacted by vehicle operation emissions and, finally, calculated the bioaccessibility-based health risk. The results indicated a significant accumulation of trace metal(loid)s in road dust, with the highest lead (Pb), cadmium (Cd), and antimony (Sb) concentrations in the garage entrance area due to higher vehicle volumes, frequent vehicle starts and stops, and lower speeds. Aerial roots exposed to hill start conditions exhibited the highest Pb, Zn, and Sb levels, potentially caused by high road dust resuspension, confirming that this tree tissue is an appropriate bioindicator. Sequential extraction revealed high percentages of carbonate-, Fe/Mn oxide-, and organic/sulphide-associated fractions of Pb, copper (Cu), and zinc (Zn) in road dust, while most Cd, Cr, Ni, and Sb occurred as residual fractions. According to the potential mobilizable fractions in sequential extraction, the in vitro gastrointestinal method could be more suitable than the physiologically based extraction test to evaluate the bioaccessibility-related risk of traffic-impacted road dust. The bioaccessibility-based health risk assessment of the road dust or soil confirmed no concern about noncarcinogenic risk, while the major risk originated from Pb although leaded gasoline was prohibited before the venue establishment. Furthermore, the cancer risks (CRs) analysis showed the probable occurrence of carcinogenic health effects from Cd and Ni to adults and from Cd, Cr, and Ni to children. Furthermore, the Cd and Pb concentrations in the edible leaves of cabbage and radish growing in gardens were higher than the recommended maximum value. This study focused on the health risks of road dust directly impacted by vehicle emissions and provides accurate predictions of trace metal(loid) contamination sources in the urban environment.

Modification of Natural Peat for Removal of Copper Ions from Aqueous Solutions

This study aimed at estimating peat adsorption properties for copper ion removal from aqueous solutions during peat modification. Two peat modifications have been studied using batch tests and quantitatively reproduced with instrumental analysis by using spectrometric, potentiometric, and thermodynamic modeling methods. The first variation—mechanical activation—was carried out in a planetary mill; for the second one—mechanochemical activation—dry sodium percarbonate (Na2CO3∙1.5H2O2) was added. The adsorption of copper ions was studied in the concentration range from 10–150 mg/L with an interaction time from 0.25–12 h. Both modifications led to significant changes in the interaction energy in the adsorption layer; thus, the acceptor properties of macromolecules were enhanced from natural peat to mechanically activated peat and mechanochemically activated peat. FTIR spectra, specific surface area characteristics, and sorption experiments show the predominantly chemical nature of copper sorption. Maximum adsorption capacity was determined to be 24.1, 42.1, and 16.0 mg/g for natural peat, mechanically activated peat, and mechanochemically activated peat, respectively. The example of peat mechanochemically oxidized with Na2CO3∙1.5H2O2 shows that the improvement in the physicochemical properties (CBET and specific surface area) plays a smaller role in the sorption capacity in relation to copper ions than the presence of phenolic and carboxyl groups, the content of which decreases during oxidation.

Speciation of heavy metals in soils and their immobilization at micro-scale interfaces among diverse soil components

Heavy metal (HM) pollution of soils is a globally important ecological and environmental problem. Previous studies have focused on i) tracking pollution sources in HM-contaminated soils, ii) exploring the adsorption capacity and distribution of HMs, and iii) assessing phyto-uptake of HMs and their ecotoxicity. However, few reviews have systematically summarized HM pollution in soil-plant systems over the past decade. Understanding the mechanisms of interaction between HMs and solid soil components is consequently key to effectively controlling and remediating HM pollution. However, the compositions of solid soil phases are diverse, their structures are complex, and their spatial arrangements are heterogeneous, all leading to the formation of soil micro-domains that exhibit different particle sizes and surface properties. The various soil components and their interactions ultimately control the speciation, transformation, and bioavailability of HMs in soils. Over the past few decades, the extensive application of advanced instrumental techniques and methods has greatly expanded our understanding of the behavior of HMs in organic mineral assemblages. In this review, studies investigating the immobilization of HMs by minerals, organic compounds, microorganisms, and their associated complexes are summarized, with a particular emphasis on the interfacial adsorption and immobilization of HMs. In addition, methods for analyzing the speciation and distribution of HMs in aggregates of natural soils with different particle sizes are also discussed. Moreover, we also review the methods for speciating HMs at mineral-organic micro-scale interfaces. Lastly, developmental prospects for HM research at inorganic-organic interfaces are outlined. In future research, the most advanced methods should be used to characterize the interfaces and in situ characteristics of metals and metal complexes. In particular, the roles and contributions of microorganisms in the immobilization of HMs at complex mineral-organic interfaces require significant further investigation.

Ecological risk assessment of trace metals in sediments under reducing conditions based on isotopically exchangeable pool

Determination of potential mobility of toxic trace metals in sediments under changing redox condition is important in ecological risk assessment. Current methods are limited in risk prediction in such dynamic environment. In this study, we have discussed the general disagreement from widely used methods (sediment quality guideline (SQGs), potential ecological risk index (PERI), risk assessment code (RAC) using BCR fraction information). In addition, the stable isotopic dilution method (IDM) was also modified to quantify metal lability in a microcosm experiment mimicking river bank sediment turning into anaerobic. The isotopically exchangeable Cd, Cu, Pb, and Zn quantified by IDM (%E _incub) was used in the RAC to reveal the trend of risk during this process. Strong risks from Cd are suggested by the PERI and RAC as a result of high toxicity and mobility of the element, while SQGs suggests medium risk for Cu, Pb, and Zn in certain samples. The disagreement between the results of RAC assessed by metal lability (%E _dry) and by BCR metal fractionation reflects the effect of sediment properties and source of metal contamination. The RAC based on the non-residual fractions is likely to overestimate the potential risk for most metals even there is a significant change in sediment Eh. The RAC assessed by %E _incub reveals that the variability in risk in response to the reducing Eh is not consistent. Large fluctuation in %E _incub for Cd (28.5%, 49.5%), Pb (27.6%, 18.2%), and Cu (14.4%, 24.7%) can shift the risks to a higher level in certain range of Eh in two sediments. In sediment with lower contents of metal binding phases (e.g. mineral oxides, organic matters), the release of metals can be more significant, thus higher ecological risk in changing redox condition.

Microbial Signatures in Deep CO2-Saturated Miocene Sediments of the Active Hartoušov Mofette System (NW Czech Republic)

The Hartoušov mofette system is a natural CO₂ degassing site in the central Cheb Basin (Eger Rift, Central Europe). In early 2016 a 108 m deep core was obtained from this system to investigate the impact of ascending mantle-derived CO₂ on indigenous deep microbial communities and their surrounding life habitat. During drilling, a CO₂ blow out occurred at a depth of 78.5 meter below surface (mbs) suggesting a CO₂ reservoir associated with a deep low-permeable CO₂-saturated saline aquifer at the transition from Early Miocene terrestrial to lacustrine sediments. Past microbial communities were investigated by hopanoids and glycerol dialkyl glycerol tetraethers (GDGTs) reflecting the environmental conditions during the time of deposition rather than showing a signal of the current deep biosphere. The composition and distribution of the deep microbial community potentially stimulated by the upward migration of CO₂ starting during Mid Pleistocene time was investigated by intact polar lipids (IPLs), quantitative polymerase chain reaction (qPCR), and deoxyribonucleic acid (DNA) analysis. The deep biosphere is characterized by microorganisms that are linked to the distribution and migration of the ascending CO₂-saturated groundwater and the availability of organic matter instead of being linked to single lithological units of the investigated rock profile. Our findings revealed high relative abundances of common soil and water bacteria, in particular the facultative, anaerobic and potential iron-oxidizing Acidovorax and other members of the family Comamonadaceae across the whole recovered core. The results also highlighted the frequent detection of the putative sulfate-oxidizing and CO₂-fixating genus Sulfuricurvum at certain depths. A set of new IPLs are suggested to be indicative for microorganisms associated to CO₂ accumulation in the mofette system.

Phytostabilization of Pb-Zn Mine Tailings with Amorpha fruticosa Aided by Organic Amendments and Triple Superphosphate

A greenhouse pot trial was conducted to investigate the effect of organic amendments combined with triple superphosphate on the bioavailability of heavy metals (HMs), Amorpha fruticosa growth and metal uptake from Pb-Zn mine tailings. Cattle manure compost (CMC), spent mushroom compost (SMC) and agricultural field soil (AFS) were applied to tailings at 5%, 10%, 20% and 30% w/w ratio, whereas sewage sludge (SS) and wood biochar (WB) were mixed at 2.5%, 5%, 10% and 20% w/w ratio. Triple superphosphate (TSP) was added to all the treatments at 4:1 (molar ratio). Amendments efficiently decreased DTPA-extracted Pb, Zn, Cd and Cu in treatments. Chlorophyll contents and shoot and root dry biomass significantly (p < 0.05) increased in the treatments of CMC (except T4 for chlorophyll b) and SMC, whereas treatments of SS (except T1 for chlorophyll a and b), WB and AFS (except T4 for chlorophyll a and b) did not show positive effects as compared to CK1. Bioconcentration factor (BCF) and translocation factor (TF) values in plant tissues were below 1 for most treatments. In amended treatments, soluble protein content increased, phenylalanine ammonialyase (PAL) and polyphenol oxidase (PPO) decreased, and catalase (CAT) activity showed varied results as compared to CK1 and CK2. Results suggested that A. fruticosa can be a potential metal phytostabilizer and use of CMC or SMC in combination with TSP are more effective than other combinations for the in situ stabilization of Pb-Zn mine tailings.

Spatio-temporal variations of sedimentary metals in a large suburban lake in southwest China and the implications for anthropogenic processes

Environment quality of suburban and urban lakes receives special attention due to their great impacted by human perturbations and important roles in ecosystem services. Herein, the spatio-temporal variations of 10 metal and metalloid elements in 13 sediment cores from a large suburban lake (Dianchi) were studied to explore the changes in sedimentary environment and pollution and their associations with human activities since the last century. Concentrations of each element were largely varied at spatial scales, but showed similar vertical trends among the profiles, suggesting comparable changes in sedimentary processes in each lake region. Cluster analysis showed two groups of elements: group I includes Al, Ti, Cu, Cr and Ni, and group II includes As, Cd, Hg, Pb and Zn. Temporally, concentrations of all elements were generally constant until the 1950s. Thereafter, group I elements along with the clay percentage started to decrease, indicating accelerated input of coarser soils due to strengthening human perturbation and changing land use. However, group II elements showed increasing values of concentrations, particularly the enrichment factors (EF = 1.0-10.8), which peaked between mid-1990s and 2000, indicating continued pollutants input with watershed economic development. With the implementation of environment management measures, pollution was initially restrained or reduced in recent decades as indicated by the stable EFs and sedimentary fluxes of Cd, Hg, Pb and Zn and decreasing values of As. Spatially, the stocks of anthropogenic As, Cd, Hg, Pb and Zn were higher in the northern while lower in the southern lake area. This spatial difference was mainly due to the large input of industrial and domestic wastewaters in the northern compared to the area in the southern that receives runoff from agricultural and forested land. Overall, the spatio-temporal patterns in accumulation of metal and metalloid elements in the lake reliably reflected the impacts of watershed human activities.

Heavy metal behaviour at mineral-organo interfaces: Mechanisms, modelling and influence factors

The mineral-organo composites control the speciation, mobility and bioavailability of heavy metals in soils and sediments by surface adsorption and precipitation. The dynamic changes of soil mineral, organic matter and their associations under redox, aging and microbial activities further complicate the fate of heavy metals. Over the past decades, the wide application of advanced instrumental techniques and modelling has largely extended our understanding on heavy metal behavior within mineral-organo assemblages. In this review, we provide a comprehensive summary of recent progress on heavy metal immobilization by mineral-humic and mineral-microbial composites, with a special focus on the interfacial reaction mechanisms of heavy metal adsorption. The impacts of redox and aging conditions on heavy metal speciations and associations with mineral-organo complexes are discussed. The modelling of heavy metals adsorption and desorption onto synthetic mineral-organo composites and natural soils and sediments are also critically reviewed. Future challenges and prospects in the mineral-organo interface are outlined. More in-depth investigations are warranted, especially on the function and contribution of microorganisms in the immobilization of heavy metals at the complex mineral-organo interface. It has become imperative to use the state-of-the-art methodologies to characterize the interface and develop in situ analytical techniques in future studies.