Transport and fate of Cu and Cd in contaminated paddy soil under acid mine drainage

Environmental colloid behaviors of humic acid - Cadmium nanoparticles in aquatic environments

Humic acid (HA), a principal constituent of natural organic matter (NOM), manifests ubiquitously across diverse ecosystems and can significantly influence the environmental behaviors of Cd(II) in aquatic systems. Previous studies on NOM-Cd(II) interactions have primarily focused on the immobilization of Cd(II) solids, but little is known about the colloidal stability of organically complexed Cd(II) particles in the environment. In this study, we investigated the formation of HA-Cd(II) colloids and quantified their aggregation, stability, and transport behaviors in a saturated porous media representative of typical subsurface conditions. Results from batch experiments indicated that the relative quantity of HA-Cd(II) colloids increased with increasing C/Cd molar ratio and that the carboxyl functional groups of HA dominated the stability of HA-Cd(II) colloids. The results of correlation analysis between particle size, critical aggregation concentration (CCC), and zeta potential indicated that both Derjaguin-Landau-Verwey-Overbeek (DLVO) and non-DLVO interactions contributed to the enhanced colloidal stability of HA-Cd(II) colloids. Column results further confirmed that the stable HA-Cd(II) colloid can transport fast in a saturated media composed of clean sand. Together, this study provides new knowledge of the colloidal behaviors of NOM-Cd(II) nanoparticles, which is important for better understanding the ultimate cycling of Cd(II) in aquatic systems.

Facilitated transport of cadmium by biochar colloids aged with ultraviolet-irradiation in saturated paddy soils

Little is known about the transport of heavy metals such as cadmium (Cd(II)) with aged biochar colloids in natural soils. Herein, we investigated the cotransport behaviors of Cd(II) with ultraviolet-irradiation aged biochar colloids pyrolyzed at 400 °C (ABC400) and 700 °C (ABC700) in saturated paddy soils. Pristine biochar colloids were included for comparison. Our results showed that Cd(II) transport was significantly facilitated by pristine and aged biochar colloids in saturated paddy soils, compared to the negligible breakthrough of Cd(II) without biochar colloids. This is likely because biochar colloids acted as vehicles carrying adsorbed Cd(II) during cotransport. Compared with pristine biochar colloids, the aged biochar colloids (especially ABC400) exhibited a greater enhancement effect, with 1.4-3.7 times Cd(II) transport in soils, likely due to stronger sorption affinity and higher mobility of aged biochar colloids towards Cd(II). Synergistic transport of aged biochar colloids with Cd(II) was relatively lower in the red soil than that in the huangni soil, probably related to the higher content of iron oxides, larger specific surface area, and lower content of soil organic matter in the red soil. A two-site kinetic retention model was employed to successfully simulate the cotransport of aged biochar colloids with Cd(II) in paddy soils. Our findings illustrate that light irradiation could accelerate the mobility of biochar colloids, as well as their synergistic carrier of Cd(II). This could trigger the potential cotransport risks when biochar is applied for field remediation of Cd-contaminated soils over a long period of time.

Heavy metals release in lead-zinc tailings: Effects of weathering and acid rain

Heavy metals (HMs) release from lead (Pb)-zinc (Zn) tailings poses significant environmental risks to surrounding areas. Furthermore, with the natural weathering and frequently happened acid rain events, the release of HMs could be elevated. This study conducted a series of laboratory column experiments with thermodynamics and hydrogeochemical analysis to investigate the environmental behavior of HMs release in Pb-Zn tailings under natural weathering conditions and acid rain events. Results showed that the weathering of calcite facilitates the release of Pb (17.9 mg/kg) and cadmium (Cd) (0.15 mg/kg), while acid rain promotes Zn release (10.5 mg/kg) from the Fe-Mn oxides, with no significant change for arsenic (As). Among the influencing factors during the column experiments, the oxidation-reduction potential (ORP) was identified as the primary indicator for the predictions of the HMs release behavior based upon the Random Forest model (R2 = 0.973 - 0.997). Correlation analysis revealed a strong relationship between coexistent ions and HM release patterns. Therefore, saturation index (SI) could effectively identify the influence range of each mineral phase on HM release. This study provides scientific evidence for effective management in carbonate-type tailings ponds.

Polyethylene nanoplastics, tebuconazole and cadmium affect soil-wheat system by altering rhizosphere microenvironment under single or combined exposure

Microplastics and nanoplastics (NPs) are pollutants of global concern. However, the understanding of the combined effects of NPs and other pollutants in the soil-plant system remains limited, particularly for polyethylene (PE), the primary component of agricultural films. This study investigated the effects of PE NPs (0.5 %, w/w), fungicide tebuconazole (Te, 10 mg·kg-1), and cadmium (Cd, 4.0 mg·kg-1) on the soil-wheat system under single and combined exposures. The synergistic toxicity observed between NPs and Te impacted the nutritional conditions and antioxidant mechanisms of the soil-wheat system. The NPs increased the concentration of Cd in roots and the proportion of bioavailable Cd, exacerbating oxidative stress in wheat and inhibiting biomass. The soil-wheat system responded to stress by upregulating or downregulating pathways related to carbohydrate, amino acid, and sugar metabolism under various treatments. Sixteen functional genes associated with carbohydrate metabolism, amino acid metabolism, energy utilization, and gene repair at KEGG level 3 were employed to sustain microenvironmental homeostasis. Correlation analysis between microorganisms and environmental factors showed that various PGPG played roles in maintaining the health of the soil-wheat system. These results help to elucidate the comprehensive effects of NPs with other pollutants on the soil-plant system and provide new perspectives for toxic mechanisms.

Copper stress in rice: Perception, signaling, bioremediation and future prospects

Copper (Cu) is an indispensable micronutrient for plants, animals, and microorganisms and plays a vital role in different physiological processes. However, excessive Cu accumulation in agricultural soil, often through anthropogenic action, poses a potential risk to plant health and crop productivity. This review article provided a comprehensive overview of the available information regarding Cu dynamics in agricultural soils, major sources of Cu contamination, factors influencing its mobility and bioavailability, and mechanisms of Cu uptake and translocation in rice plants. This review examined the impact of Cu toxicity on the germination, growth, and photosynthesis of rice plants. It also highlighted molecular mechanisms underlying Cu stress signaling and the plant defense strategy, involving chelation, compartmentalization, and antioxidant responses. This review also identified significant areas that need further research, such as Cu uptake mechanism in rice, Cu signaling process, and the assessment of Cu-polluted paddy soil and rice toxicity under diverse environmental conditions. The development of rice varieties with reduced Cu accumulation through comprehensive breeding programs is also necessary. Regulatory measures, fungicide management, plant selection, soil and environmental investigation are recommended to prevent Cu buildup in agricultural lands to achieve sustainable agricultural goals.

Hydrochemical characteristics of abandoned coal mines derived acid mine drainage in a typical karst basin (Wuma river basin, Guizhou China)

The hydrochemical characteristics of acid mine drainage (AMD) were investigated in Wuma River Basin, China. AMD was sampled from nine closed coal mine (CCM) sites to study the temporal and spatial evolution of pH, dissolved oxygen (DO), electrical conductivity (ED), total hardness (THR), total dissolved salt (TDS), and trace elements. The surface water (river) and groundwater surrounding mine sites were sampled to evaluate the potential pollution derived from AMDs. The TDS content of AMD was higher than that of surface water and groundwater. The dominant factors influencing TDS were the pH, temperature, and wet or dry season (which played a role in controlling microbial activity), HCO3 - balance, and REDOX during the evolutionary process. The hydrochemical type of AMD was dependent on the evolutionary stage. From observations, most AMDs were in the form of the SO4 2--Ca2+•Mg2+ type that was characterized by a low pH, low [HCO3 -], high [SO4 2-], and high [Fe]. In addition, the AMD samples were undergoing stage I and II processes, in which SO4 2- and trace elements were generated. The surface water and groundwater were primarily classified as the HCO3 --Ca2+•Mg2+ type, which accounted for their self-cleaning capacity, as indicated by the high [HCO3 -]. The surface water and groundwater could be affected by the surrounding AMD depending on the geographical location. The surface water and groundwater sites that were located downstream of subsurface and surface runoff were obviously affected by AMD. After being polluted by AMD, surface water and groundwater contained higher levels of trace elements and emerged as the HCO3 -•SO4 2--Ca2+•Mg2+ type.

Biogeochemical cycling in paddy soils controls antimony transformation: Roles of iron (oxyhydr)oxides, organic matter and sulfate

In paddy fields, periodic flooding and drainage phases can significantly affect the availability of antimony (Sb), but the underlying mechanisms remain unclear. In this study, Sb-contaminated paddy soil was incubated under anaerobic (40 day) and subsequently aerobic (40-55 day) conditions. The Sb fractions was investigated and a kinetic model was established to quantitatively evaluate the main processes controlling Sb transformation. Under anaerobic conditions, the reductive dissolution of iron (Fe) (oxyhydr)oxides, the release of soil colloids, and dissolved organic carbon (DOC) could facilitate the release of Sb(V), while newly released Sb(V) were synchronously reduced to Sb(III) that could be incorporated into the solid phase (34.1%, 40 day) or precipitated as Sb2S3 (9.7%, 40 day). After soil aeration, a significant increase in dissolved and extracted Sb(V) (34.7%, 45 day) was observed due to the Sb(III) oxidization by the reactive oxygen species (ROS) generated from Fe(II) oxidization. The dissolved and extracted Sb(V) were simultaneously incorporated into the solid phase as the re-aggregation of soil colloids and DOC, and only contributed to 17.1% of the total Sb content at the end of aerobic phase (55 day). Our results elucidated the mechanisms about how biogeochemical Fe/S/C cycling jointly controlled Sb transformation in paddy systems.

Complex electrical measurements of waste rock during acid mine drainage generation and release: Kinetic column tests

Acid mine drainage (AMD) emanating from waste rock piles (WRPs) at mining sites is a global concern. Successful rehabilitation of these sites requires effective characterization and monitoring of the waste rock during AMD generation/release. Traditional approaches involve ex-situ analysis of waste rock and porewater samples collected via corings and monitoring wells; however, this is highly disruptive, costly, and provides sparsely distributed point information across enormous volumes typical of WRPs. Geoelectrical techniques are a promising approach for non-invasive continuous imaging; however, their application has been limited to 'one-off' imaging with few studies on monitoring waste rock evolution. The objective of this study is to assess the geoelectrical signatures of changing waste rock during AMD generation/release. Field waste rock samples were extracted from three mine WRPs and first characterized for mineralogy and acid generation potential. Kinetic tests were then performed on each sample using leaching columns and humidity cells, with simultaneous measurements of effluent quality and complex electrical conductivity (real and imaginary components measure conduction and polarization, respectively). Results show that real conductivity was highly sensitive to changes associated with AMD leachate quality (e.g., 28,800 to 68 mg/L acidity) and surface of the waste material. Imaginary conductivity measurements identified changes in the waste mineralogy over time, though these signatures were not very distinct, which is likely due to low sulfide contents and limited oxidation (e.g., 0.59 wt% sulfide and 33% air saturation). This study improves our understanding of geoelectrical signatures associated with real waste rock, demonstrating the potential application of the electrical resistivity tomography and induced polarization techniques for mine waste investigations.