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

Challenges of Using Whole-Cell Bioreporter for Assessment of Heavy Metal Bioavailability in Soil/Sediment

Soil and sediment contamination with heavy metals (HMs) is a critical environmental issue, posing significant risks to both ecosystems and human health. Whole-cell bioreporter (WCB) technology offers a promising alternative to traditional detection techniques due to its ability to rapidly assess the bioavailability of pollutants. Specifically, lights-on WCBs quantify pollutant bioavailability by measuring bioluminescence or fluorescence in response to pollutant exposure, demonstrating comparable accuracy to traditional methods for quantitative pollutant detection. However, when applied to soil and sediment, the signal intensity directly measured by WCBs is often attenuated due to interference from solid particles, leading to the underestimation of bioavailability. Currently, no standardized method exists to correct for this signal attenuation. This review provides a critical analysis of the benefits and limitations of traditional detection methods and WCB technology in assessing HM bioavailability in soil and sediment. Based on the approaches used to address WCB signal attenuation, correction methods are categorized into four types: the assumed negligible method, the non-inducible luminescent control method, the addition of a standard to a reference soil, and a pre-exposure bioreporter. We provide a comprehensive analysis of each method's applicability, benefits, and limitations. Lastly, potential future directions for advancing WCB technology are proposed. This review seeks to establish a theoretical foundation for researchers and environmental professionals utilizing WCB technology for pollutant bioavailability assessment in soil and sediment.

Iron, Sulfur, and Carbon Dynamics Collectively Regulate the Fate of Cadmium over the Sulfidation-Reoxidation Cycle

Cadmium bioavailability is sensitive to redox fluctuations, with its fate linked to the coupled dynamics of Fe, S, and C. This study examines the behavior of Cd-loaded ferrihydrite (Fh) with/without organic matter (OM) undergoing S(-II)-induced reduction followed by O2-induced reoxidation. During sulfidation, S(-II) was fully consumed, and Fh was partially reduced to Fe(II) species, with some OM released from the Fh surface. Meanwhile, Cd initially adsorbed on Fh was completely converted to CdS, regardless of Cd loading or the presence of OM. Upon reoxidation, Fe(II) species were reoxidized to Fe(III) oxides, which recaptured OM, while solid-phase S(-II) was oxidized to S0 and sulfate. Concurrently, partial oxidation of CdS occurred, mainly driven by H2O2 generated during Fe(II) oxidation, with minor contributions from •OH and O2, but OM inhibited CdS oxidation, primarily by scavenging H2O2. Released Cd from CdS oxidation was predominantly readsorbed on Fe(III) oxides. Additionally, released Cd was partially structurally incorporated into newly formed Fe(III) oxides while some CdS was encapsulated within Fe(III) oxide aggregates. However, OM interactions with Fe(III) oxides reduced the formation of these Cd species. These findings provide insights into the molecular-scale mechanisms governing Cd dynamics in redox-dynamic environments.

Immobilization or mobilization of heavy metal(loid)s in lake sediment-water interface: Roles of coupled transformation between iron (oxyhydr)oxides and natural organic matter

Iron (Fe) (oxyhydr)oxides and natural organic matter (NOM) are active substances ubiquitously found in sediments. Their coupled transformation plays a crucial role in the fate and release risk of heavy metal(loid)s (HMs) in lake sediments. Therefore, it is essential to systematically obtain relevant knowledge to elucidate their potential mechanism, and whether HMs provide immobilization or mobilization effect in this ternary system. In this review, we summarized (1) the bidirectional effect between Fe (oxyhydr)oxides and NOM, including preservation, decomposition, electron transfer, adsorption, reactive oxygen species production, and crystal transformation; (2) the potential roles of coupled transformation between Fe and NOM in the environmental behavior of HMs from kinetic and thermodynamic processes; (3) the primary factors affecting the remediation of sediments HMs; (4) the challenges and future development of sediment HM control based on the coupled effect between Fe and NOM from theoretical and practical perspectives. Overall, this review focused on the biogeochemical coupling cycle of Fe, NOM, and HMs, with the goal of providing guidance for HMs contamination and risk control in lake sediment.

Reducing cadmium and arsenic accumulation in rice grains: The coupled effect of sulfur's biomass dilution and soil immobilization analyzed using meta-analysis and machine learning

The biogeochemical cycling of sulfur (S) in paddy soil influences cadmium (Cd) and arsenic (As) migration. However, the impact of S application on Cd and As within the soil-rice system has not been fully explored. This study aimed to examine the effect of S application on Cd/As soil-rice system dynamics by conducting an extensive meta-analysis of 322 sets of observational data from 46 publications, which were published between 2004 and 2023. Furthermore, a machine learning model was only used to forecast the potential influence of S on Cd within the soil-rice system rather than the influence of As due to the limited data samples. The results indicated that the basal application of S mainly reduced the accumulation of Cd and As in the grains [Cd: 29.00 % (28.48 % to 29.52 %); As: 38.31 % (37.79 % to 38.85 %)] by the coupling effect of promoting rice growth 40.87 % (40.61 %-41.14 %) and reducing the soil bioavailable Cd/As by 18.20 % (18.05 % to 18.36 %)/19.59 % (19.44 % to 19.75 %). However, the efficacy of actual field farmland remediation is often suboptimal because the actual soil physical and chemical properties frequently do not meet the ideal conditions [pH: 6.5-7.5, Total S: < 200 mg/kg, soil organic matter (SOM): 30-40 g/kg, Total Fe: 20-30 g/kg] that are required to mitigate Cd and As accumulation in rice grains. Notably, the random forest machine learning model achieved an acceptable level of accuracy when compared to the excessive linear regression simulation. The model suggested that the decrease in the Cd/As accumulation in the rice was due to the soil available S content, which was primarily influenced by S application. This study provides novel insights for managers and researchers for the amelioration of Cd/As-contaminated farmland soil.

Migration and availability of Ni and Cd in industrial soils under different leaching conditions: Insights from DGT and DIFS models

Rainfall runoff can mobilize heavy metals in industrial soils, posing environmental risks. The mobility and distribution of heavy metals in different industrial soil layers are often overlooked. This study employed dynamic leaching experiments in layered soil columns with DGT (the diffusive gradients in thin films) measurements and DIFS (DGT-induced fluxes in soils and sediments) model to describe the migration, availability, and resupply ability of metals at different depths in surface and deep soil columns of industrial soils. Results showed significantly higher available concentrations (CDGT and CSoln) of Ni and Cd in surface soils compared to deep soils, likely due to the differences in soil physiochemical properties (contamination, pH, and soil texture). Continuous leaching promoted the migration of available Ni and Cd in surface soils. Maximum values of RNi (0.79-0.91) and RCd (0.75-0.80) were observed in the top layer (0-4 cm) of the surface soil, consistent with the trends of RFe. Combined DGT and DIFS model analysis implied higher potential availability and resupply of Ni and Cd in surface soil columns. These findings highlight the importance of considering dynamic leaching effects on heavy metal transport, availability, and release in industrial soils.

A new simple index for characterizing the labile heavy metal concentration in soil by diffusive gradients in thin films technique

Diffusive gradients in thin films technique (DGT) is recognized as a more reliable method for determining labile heavy metal (HM) concentration in soil than traditional destructive methods. However, the current DGT measurement index, CDGT, theoretically underestimates the true labile concentration (Clabile) of HMs in soil and lacks direct comparability with the conventional soil HM content indices due to unit differences. Here, we proposed CDGT-W, a new simple index which is defined as the HM accumulation in the binding layer, normalized to the weight of soil (optimized water content = 100% of the maximum water holding capacity) filled in the open cavity-type DGT device over a specified deployment time (optimized time = 24 h). The procedure for measuring CDGT-W is analogous to that of CDGT but includes precise determination of water content (water/dry soil) and the mass of soil filled in the cavity. We conducted measurements of Cu, Pb, Cr(Ⅵ) and As(V) as CDGT-W, CDGT, solution concentration (Csoln), and CaCl2 extractable concentration (CCaCl2) on three soils with a diverse range of HM concentrations. CDGT-W showed significant linear correlations with all other tested indexes. The ratios of CDGT-W to CCaCl2 varied between 0.30 and 0.98 for all HM-soil combinations with only one exception, a range much greater than CDGT/Csoln (typically <0.1) but lower than 1. This suggested that CDGT-W may more accurately reflect Clabile than CDGT (theoretically underestimates Cliable) and CCaCl2(likely overestimates Cliable). Additionally, CDGT-W measurements for these four HMs exhibited a broad measure concentration range and a low detection limit (mg/kg level). Consequently, CDGT-W may offer a more reliable alternative to CDGT for characterizing Clabile in unsaturated soils.

Use of diffusive gradients in thin-film technique to predict the mobility and transfer of nutrients and toxic elements from agricultural soil to crops-an overview of recent studies

The purpose of this review was to survey the recent applications of the diffusive gradients in thin films (DGT) technique in the assessment of mobility and bioavailability of nutrients and potentially toxic elements (PTEs) in agricultural soil. Many studies compared the capabilities of the DGT technique with those of classical soil chemical extractants used in single or sequential procedures to predict nutrients and PTE bioavailability to crops. In most of the published works, the DGT technique was reported to be superior to the conventional chemical extraction and fractionation methods in obtaining significant correlations with the metals and metalloids accumulated in crops. In the domain of nutrient bioavailability assessment, DGT-based studies focused mainly on phosphorous and selenium labile fraction measurement, but potassium, manganese, and nitrogen were also studied using the DGT tool. Different DGT configurations are reported, using binding and diffusive layers specific for certain analytes (Hg, P, and Se) or gels with wider applicability, such as Chelex-based binding gels for metal cations and ferrihydrite-based hydrogels for oxyanions. Overall, the literature demonstrates that the DGT technique is relevant for the evaluation of metal and nutrient bioavailability to crops, due to its capacity to mimic the plant root uptake process, which justifies future improvement efforts.

Impact of iron and sulfur cycling on the bioavailability of cadmium and arsenic in co-contaminated paddy soil

The biogeochemical cycling of iron (Fe) or sulfur (S) in paddy soil influences the cadmium (Cd) and arsenic (As) migration. However, the influence of coupled reduction effects and reaction precedence of Fe and S on the bioavailability of Cd and As is still not fully understood. This study aimed to reveal the influence of Fe and S reduction on soil Cd and As mobility under various pe + pH conditions and to elucidate the related mechanism in subtropical China. According to the findings, higher adsorption from Fe reduction caused high-crystalline goethite (pe + pH > 2.80) to become amorphous ferrihydrite, which in turn caused water-soluble Cd (62.0%) to first decrease. Cd was further decreased by 72.7% as a result of the transformation of SO42- to HS-/S2- via sulfate reduction and the formation of CdS and FeS. As release (an increase of 8.1 times) was consequently caused by the initial reduction and dissolution of iron oxide (pe + pH > 2.80). FeS had a lesser impact on the immobilization of As than sulfate-mediated As (V) reduction in the latter stages of the reduction process (pe + pH < 2.80). pe + pH values between 3 and 3.5 should be maintained to minimize the bioavailability of As and Cd in moderate to mildly polluted soil without adding iron oxides and sulfate amendments. The practical remediation of severely co-contaminated paddy soil can be effectively achieved by using Fe and S additions at different pe + pH conditions. This technique shows promise in reducing the bioavailability of Cd and As.

Oxidative compensation mechanism of Fe-S synergetic inhibition of Cd activity in paddy field during flooding and drainage

It is known that the transformation of Fe and S forms in soil affects the migration and activity of Cd, but the coordinated regulation of Cd activity by Fe and S under different redox conditions is still unclear. Here, Diffusive gradients in thin films (DGT), an in-situ monitoring technique, is used to explore the difference of the regulation of Cd activity in paddy fields with ferrihydrite (FH) and ferrihydrite coprecipitated by sulfate (FH-S) under the flooding and drainage conditions. The addition of FH-S and FH significantly reduced the activity of Cd (Dissolved, Exchanged, and CDGT-Cd). Compared with pure FH, the adsorption extent of Cd in FH was enhanced by increasing concentrations of SO42- (i.e., S/Fe ratio), which is attributed to the decrease in the crystallinity of FH by sulfate. During soil flooding, the addition of FH-S promoted the production of metal sulfide (CdS and FeS/FeS2). The activity of Cd increased after drainage, while the FH-S treatment groups delayed the release of Cd. After 30 days of drainage, the concentration of Cd in FH-S treatment groups decreased by 28.9-44.1 % compared with the control group. The fresh FeS/FeS2 is not the main adsorbent for fixing Cd, and due to the existence of oxidation compensation mechanism, the preferential oxidation of FeS/FeS2 delays the release of Cd in the drainage stage. Our study shed new light on the mechanism of Fe-S synergistic regulation of Cd and remediation of Cd-contaminated soils.