The toxicity of hexavalent chromium to soil microbial processes concerning soil properties and aging time

Influence of soil properties and aging on exogenous antimony toxicity to Caenorhabditis elegans in agricultural soil

Exploring the influence of soil on antimony (Sb) aging could help predict Sb toxicity on nematodes that play an important role in agricultural soil nitrogen cycling. This study aimed to investigate the major soil factors affecting the aging process and toxicity of exogenous Sb. Therefore, nematodes were exposed to varying levels of Sb contamination (0-6400 mg/kg) in nine agricultural soils, with aging periods of 7, 56, and 168 days, under dark conditions at 20 ± 0.5 °C for 96 h. The results suggested that nematode reproduction was more sensitive to the toxicity of exogenous trivalent Sb (Sb(III)) compared to growth and fertility. Following 7-168 days of aging, the EC50 of nematode reproduction increased from 546-1557 to 3560-6193 mg/kg in nine soils contaminated by exogenous Sb(III). Exogenous Sb(III) toxicity is overestimated without considering its aging process. The aging factors (AF) of nine soils aged over 7-168 days were calculated as 3.54-8.03. The regression equation AF = 0.923 pH - 0.812 (n = 9, adjust-r2 = 0.687, P = 0.004) indicated that pH was the primary soil factor explaining 85.2% of the variance in the aging process of exogenous Sb(III). No significant toxicity was observed in soils contaminated with exogenous pentavalent Sb after 7 days of aging. These findings could provide guidance for the adjustment of Sb toxicity data, the revision of soil environmental quality standard, and efficient soil environmental management.

Environmental risk thresholds and prediction models of Cd in Chinese agricultural soils

Soil environmental risk threshold of cadmium (Cd) is an important index in formulating soil protection policy. Environmental risk threshold refers to the maximal allowable critical concentration of hazardous substances in the environment. Although there is less study on how to determine soil Cd environmental risk threshold, it is a crucial indicator in formulating soil conservation policies and a key factor in assessing soil environmental quality. The main research content of the study is deducing the environmental risk threshold, aiming to provide scientific basis for the study of environmental quality standards of agricultural land and provide technical support for the protection of Cd pollution of agricultural land. The hazard concentration of 5 % species (HC5, which protects 95 % of species) was determined here using different toxicological data of Cd from 23 test endpoints, interspecific extrapolation using the species sensitivity distribution (SSD) method, and a prediction model was created on the basis of several soil parameters. According to the findings, Cd effective concentration (EC10) (Cd concentration which blocks 10 % of an endpoint's bioactivity) varied from 0.109 to 221 mg·kg-1, and the hormetic response induced by Cd reached 118 % displaying in the dose-response curve of Lolium perenne L.. Toxicology data was rectified by the aging factor considering biogeochemical processes of the newly added pollutants prior to SSD curves fitting. After that, the prediction model was created with the equation of LogHC5 = 0.147 pH + 0.067 OC -1.616. The field test properly validated the prediction model, demonstrating its ability to forecast Cd toxicity levels for various soil conditions. This study offers a scientifically sound methodology for determining the environmental risk limitation for Cd and identifies critical paths for the preservation of environmental species.

Intra- and inter-habitat variation in sediment heavy metals, antibiotics and ecological risks in Mai Po RAMSAR, China

Distribution of heavy metals (HMs) and antibiotics (ABs) in surface sediments of three habitats: mudflat, mangrove and gei wai (inter-tidal shrimp ponds), at Mai Po RAMSAR were determined with inductively coupled plasma and liquid chromatograph tandem - mass spectrometry, respectively. Eight HMs (Cr, As, Pb, Cd, Mn, Ni, Cu and Zn), and ten ABs (tetracyclines, quinolones, macrolides and sulphonamides) were detected in all habitats, with relatively lower concentration in gei wai. Ecological risk assessment based on PNEC revealed that HMs posed a higher ecological risk to microorganisms than ABs. All metals except Mn were above their respective threshold effect levels according to sediment quality guidelines, indicating their potential toxicity to benthos. The enrichment factor and geo-accumulation index on background values suggested sediments were moderately polluted by Zn, Cu and Cd, possibly from anthropogenic inputs. This study implies that HMs pollution must be prevented through proper regulation of agricultural and industrial discharge.

Ecotoxicological effects of soil lithium on earthworm Eisenia fetida: Lethality, bioaccumulation, biomarker responses, and histopathological changes

Lithium is an emerging environmental contaminant in the current low-carbon economy, but little is known about its influences on soil invertebrates. In this work, earthworm Eisenia fetida was exposed to soils treated with different levels of lithium for 7 d, and multiple ecotoxicological parameters were evaluated. The results showed that mortality was dose-dependent and lithium's median lethal content (LC₅₀) to earthworm was respectively 865.08, 361.01, 139.36, and 94.95 mg/kg after 1 d, 2 d, 4 d, and 7 d exposure. The bioaccumulation factor based on measured exogenous lithium content (BF_exog) respectively reached 0.79, 1.01, 1.57, and 1.27 with the increasing lithium levels, suggesting that lithium accumulation was averagely 1.16-fold to the exogenous content, and 74.42%∼81.19%, 14.54%∼18.23%, and 2.26%∼8.02% of the lithium in exposed earthworms were respectively retained in the cytosol, debris, and granule. Then, lithium stress stimulated the activity of superoxide dismutase, peroxidase, catalase, acetylcholinesterase, and glutathione S-transferase as well as the content of 8-hydroxy-2-deoxyguanosine and metallothionein, indicating the generation of oxidative damage, while the content of reactive oxygen species and malondialdehyde decreased. Finally, lithium introduced histopathological changes, including the degenerated seminal vesicle and muscle hyperplasia, as well as high or extreme nuclear DNA damage. This study confirmed the obvious bioaccumulation and toxic effects caused by soil lithium via ecotoxicological data, providing new theoretical insights into understanding the ecological risks of lithium to soil invertebrates.

Distribution and Speciation of Heavy Metal(loid)s in Soils under Multiple Preservative-Treated Wooden Trestles

The widespread use of wood preservatives, such as chromated copper arsenate (CCA), alkaline copper quaternary (ACQ), and copper azole (CA), may cause environmental pollution problems. Comparative studies on the effect of CCA-, ACQ-, and CA-treated wood on soil contamination are rarely reported, and the behavior of soil metal(loid) speciation affected by preservatives has been poorly understood. Soils under the CCA-, ACQ-, and CA-treated boardwalks were collected to investigate metal(loid) distribution and speciation at the Jiuzhaigou World Natural Heritage site. The results showed that the maximum mean concentrations of Cr, As, and Cu were found in soils under the CCA, CCA, and CCA plus CA treatments and reached 133.60, 314.90, and 266.35 mg/kg, respectively. The Cr, As, and Cu contamination in soils within a depth of above 10 cm was high for all types of boardwalks and limited in the horizontal direction, not exceeding 0.5 m. Cr, As, and Cu in soils were mainly present as residual fractions in all profiles and increased with depth. The proportion of non-residual As in soil profiles under CCA- and CCA plus CA-treatment and exchangeable Cu in CA- and CCA plus CA-treatment were significantly higher than those in the profiles under the other preservative treatments. The distribution and migration of Cr, As, and Cu within soils were influenced by the preservative treatment of trestles, in-service time of trestles, soil properties (e.g., organic matter content), geological disasters (e.g., debris flow), and elemental geochemical behavior. With the CCA treatment for trestles successively replaced by ACQ and CA treatments, the types of contaminants were reduced from a complex of Cr, As, and Cu to a single type of Cu, achieving a reduction in total metal content, toxicity, mobility, and biological effectiveness, thus reducing environmental risks.

Ecological risk threshold for Pb in Chinese soils

Derivation of ecological risk threshold (the threshold concentration value that protect a certain proportion of species within the acceptable hazard level) of lead (Pb) is a yardstick and plays a key role in formulating soil protection policies, while the research about deducing soil Pb ecological risk threshold is still limited. In this study, toxicological data of Pb based on 30 different test endpoints was collected from our experiment and literature, and applied into interspecific extrapolation by species sensitivity distribution (SSD) method to derive the hazard concentration for 5% of species (HC₅, that can protect 95% of species), the prediction models according to different soil properties were established. The results showed that EC₁₀ (the effective concentrations of Pb that inhibit 10% of endpoint bioactivity) ranged from 205.6 to 1596.3 mg kg¹, and hormesis induced by Pb were up to 118%. Toxicity data were corrected by leaching and aging process before SSD curves fitting. HC₅ was then derived and prediction model was developed, as LogHC₅ = 0.134 pH + 0.315 LogOC + 0.324 LogCEC + 1.077. The prediction model was well verified in the field test, indicating that can correctly estimate Pb ecotoxicity thresholds in different soils. This study provides a scientific frame for deriving the ecological risk threshold of Pb and is of great significance for ecological species protection.

Soil ammonium (NH4+) toxicity thresholds for restoration grass species

Ionic rare earth mining has resulted in large amounts of bare soils, and revegetation success plays an important role in mine site rehabilitation and environmental management. However, the mining soils still maintain high NH₄⁺ concentrations that inhibit plant growth and NH₄⁺ toxicity thresholds for restoration plants have not been established. Here we investigated the NH₄⁺ toxicological effects and provided toxicity thresholds for grasses (Lolium perenne L. and Medicago sativa L.) commonly used in restoration. The results show that high NH₄⁺ concentration not only reduces the plant biomass and soluble sugars in leaves but also increases the H₂O₂ and MDA content, and SOD, POD, and GPX activities in roots. The SOD activities and root biomass can be adopted as the most NH₄⁺ sensitive biomarkers. Six ecotoxicological endpoints (root biomass, soluble sugars, proline, H₂O₂, MDA, and GSH) of ryegrass, eight ecotoxicological endpoints (root biomass, soluble sugars, proline, MDA, SOD, POD, GPX, and GSH) of alfalfa were selected to determine the threshold concentrations. The toxicity thresholds of NH₄⁺ concentrations were proposed as 171.9 (EC₅), 207.8 (EC₁₀), 286.6 (EC₂₅), 382.3 (EC₅₀) mg kg^-1 for ryegrass and 171.9 (EC₅), 193.2 (EC₁₀), 234.7 (EC₂₅), 289.6 (EC₅₀) mg kg^-1 for alfalfa. The toxicity thresholds and the relation between plant physiological indicators and NH₄⁺ concentrations can be used to assess the suitability of the investigated plants for ecological restoration strategies.

Ecological toxicity (ECx) of Pb and its prediction models in Chinese soils with different physiochemical properties

The lack of toxicological data becomes the main bottleneck of ecological risk assessment of lead (Pb) in Chinese soils. The present study assessed Pb toxicity on three underground test endpoints (barley root elongation, earthworm avoidance response, and substrate-induced respiration (SIR) of microorganism) in 10 different soils. Hormetic dose-response induced by Pb was >118 % for earthworm avoidance response. EC₁₀ and EC₅₀ (the effective concentrations of Pb that inhibit 10 % or 50 % of endpoint bioactivity and also represents the toxicity threshold of Pb) after leaching increased by 0.32-8.73 times, and 1.02-3.75 times, respectively. Leaching factor (LF) prediction models indicated pH and cation exchange capacity (CEC) were the vital predictors for LF₁₀ and LF₅₀, explaining 60.6 % and 73.1 % of variations, respectively. SIR was one sensitive test endpoint for Pb toxicity, with the lowest of EC₁₀ and EC₅₀ values (from 373.7 to 1008.5 mg·kg^-1, and from 837.1 to 2869.0 mg·kg^-1, respectively). The best prediction models between EC_x and soil properties is LogEC₅₀ = 1.324Log(pH) + 0.423Log(CEC) + 1.742 (R² = 0.761, p < 0.01). The results displayed significant implications for deriving EC_x of Pb, and provided a scientific basis for soil ecological risk assessment of Pb.

Aging factor and its prediction models of chromium ecotoxicity in soils with various properties

Aging of pollutants determines bioavailability and toxicity thresholds of environmental pollutants in soil. However, the ecotoxicity of chromium (Cr) rarely considers the effect of aging as well as soil properties. In order to explore the aging characteristics and establish their quantitative relationship with different soil properties, this study selected 7 soils with different properties through exogenous addition of Cr and determined its toxicity on barley root elongation. From 14d to 540d, EC₁₀ and EC₅₀ of barley root elongation ranged from 21.40 to 312.52 (mg·kg^-1) and 50.15 to 883.88 (mg·kg^-1) respectively. The hormesis appeared in the dose-response curve of acid soil as relative barley root elongation reached >110 % compared with the control. Extended aging time of Cr from 14d to 540d was associated with the attenuation of the toxicity of Cr, as the aging factor increased from 1.26 to 6.09 for EC₅₀, from 0.88 to 4.98 for EC₁₀. The prediction model of AF_EC50 and soil properties is lg (AF_360d) = 0.306lg Clay+0.026lg CEC + 0.240 (R² = 0.872, P < 0.01). The results demonstrated that with the extension of aging time, the toxicity of Cr decreased at 360d and reached a slow reaction stage, after that soil OC, Clay and CEC could well explain the aging procedure of Cr (VI). These results are beneficial for risk assessment of Cr contaminated soils and establishment of a soil environmental quality criteria for Cr.

Chitosan-based composite microspheres for treatment of hexavalent chromium and EBBR from aqueous solution

Hexavalent chromium is widely used in industrial fields, but its pollution has posed a great threat to the environment due to its high toxicity. We created a chitosan-based microsphere biosorbent (CP) by combining polyethyleneimine with chitosan adopting inverse emulsion polymerization method. Under the optimal conditions (pH = 3), the maximum adsorption capacity of composite microspheres can reach 299.89 mg g^-1, which is much higher than that of chitosan microspheres (168.91 mg g^-1). When the amount of CP is 0.25 g L^-1, the removal rate of 50 mg L^-1 Cr(VI) and 50 mg L^-1 Eriochrome blue-black R (EBBR) can reach 95% and 99%, respectively. The time required for CP to reach adsorption equilibrium (180 min) was significantly shorter than that of chitosan microspheres (540 min), and the adsorption rate was significantly improved. Langmuir isotherm model, pseudo-second-order kinetic model and thermodynamic calculation results penetrated an endothermic spontaneous, monolayer, and chemical adsorption process. Biomass composite microspheres CP has obvious selectivity and the adsorption capacity retention rate of CP was still 71.32% after four adsorption cycles. This work proposed an easily prepared and biomass-based microspheres for the effective removal of Cr(VI) in printing and dyeing wastewater pollution through adsorption.

Toxicity evaluation of pyraclostrobin exposure in farmland soils and co-exposure with nZnO to Eisenia fetida

Although the toxicity of pyraclostrobin (PYRA) to earthworms in artificial soil is well known, the toxicity of PYRA in farmland soils is yet to be explored in detail. Additionally, with more zinc oxide nanoparticles (nZnO) entering the soil environment, the risk of PYRA co-exposure with nZnO is increasing alarmingly. However, toxicity caused by this co-exposure of PYRA and nZnO is still unknown. Therefore, we assessed the biomarkers responses to reveal the toxicity of PYRA (0.1, 1, 2.5 mg/kg) on earthworms in farmland soils (black soil, fluvo-aquic soil, and red clay) and evaluated the biomarkers responses of Eisenia fetida exposed to PYRA (0.5 mg/kg)/PYRA+nZnO (10 mg/kg). Moreover, transcriptomic analysis was performed on E. fetida exposed to PYRA/PYRA+nZnO for 28 days to reveal the mechanism of genotoxicity. The Integrated Biomarker Responses (IBR) showed PYRA induced more severe oxidative stress and damage to E. fetida in farmland soils than that in artificial soil. The oxidative stress and damage induced by PYRA+nZnO were greater than that induced by PYRA. Transcriptomic analysis showed that PYRA and PYRA+nZnO significantly altered gene expression of both biological processes and molecular functions. These results provided toxicological data for PYRA exposure in three typical farmland soils and co-exposure with nZnO.