Soil migration of antimony and arsenic facilitated by colloids in lysimeter studies

Effect of freeze-thaw frequency plus rainfall on As and Sb metal(loid)s leaching from the solidified/stabilized soil remediated with Fe-based composite agent

The composite agent of ferrous sulfate, fly ash, and calcium lignosulfonate (FFC) can remediate the soil contaminated by As and Sb under cyclic freeze-thaw (F-T) via stabilization/solidification (S/S). However, the impact of high-frequency F-T cycles on the leaching behavior and migration of As and Sb in FFC-treated soils remains unclear. Here the leaching concentrations, heavy metal speciation (Wenzel's method), and Hydrus-1d simulations were investigated. The results showed that FFC effectively maintained the long-term S/S efficiency of arsenic remediation subject to an extended rainfall and freeze-thaw cycles, and stabilized the easily mobile form of As. The short-term S/S effect on Sb in the remediated soils suffering from F-T cycles was demonstrated in the presence of FFC. In a 20-year span, the mobility of Sb was affected by the number of F-T cycles (FT60 > FT20 > FT40 > FT0) in soil with a depth of 100 cm. As leaching progressed, FFC slowed the upward proportion of adsorbed As fractions but converted parts of the residual Sb to the form of crystalline Fe/Al (hydro) oxide. Moreover, the adsorption rate and capacity of As also preceded that of Sb. Long-term curative effects of FFC could be observed for As, but further development of agents capable of remedying Sb under cyclic F-T and long-term rainfall was needed. The predictive results on the migration and leaching behavior of heavy metals in S/S remediated soils may provide new insight into the long-term assessment of S/S under natural conditions.

Antimony and arsenic migration in a heterogeneous subsurface at an abandoned antimony smelter under rainfall

While antimony (Sb) and arsenic (As) co-contamination in subsurface soil systems due to the legacy of Sb smelting wastes has been documented, the role of inherent heterogeneity on pollutant migration is largely overlooked. Herein this study investigated Sb and As migration in a slag impacted, vertically stratified subsurface at an abandoned Sb smelter. A 2-dimensional flume was assembled as a lab-scale analogue of the site and subject to rainfall and stop-rain events. Reactive transport modeling was then performed by matching the experimental observations to verify the key factors and processes controlling pollutant migration. Results showed that rainfall caused Sb and As release from the shallow slag layer and promoted their downward movement. Nevertheless, the less permeable deeper layers limited physical flow and transport, which led to Sb and As accumulation at the interface. The re-adsorption of Sb and As onto iron oxides in the deeper, more acidic layers further retarded their migration. Because of the large difference between Sb and As concentrations, Sb re-adsorption was much less effective, which led to higher mobility. Our findings overall highlight the necessity of understanding the degree and impacts of physicochemical heterogeneity for risk exposure assessment and remediation of abandoned Sb smelting sites.

Effect of iron cyclic transformation on the natural purification of antimony in contaminated reservoirs of mines

To further understand the purification mechanism of antimony (Sb) in reservoirs, samples of stratified water and bottom interface sediment were collected in this study. The cross-flow ultrafiltration technique was used to separate the truly dissolved (<1 kDa) and colloidal (1 kDa-0.45 μm) phases of water, and two modified sequential extraction techniques were used to determine the Sb and Fe mineral forms in sediment, respectively. The results showed that the total Sb concentration could decrease from 142.2 μg/L in surface water to 98.6 μg/L at 16 m; this was contributed to by the removal of truly dissolved Sb. In comparison to particulate Sb (>0.45 μm), the formation of colloidal Sb played a greater role in the purification process. There was a positive correlation between Sb and Fe in the colloidal phase (r = 0.45, P < 0.05). The generation of colloidal Fe could be promoted by higher temperatures, pH values, DO, and DOC in the upper layer (0-5 m). However, the complexation of DOC with colloidal Fe inhibited the adsorption of truly dissolved Sb. After entering the sediment, the secondary release of Sb could not increase the Sb concentration in the lower layer obviously, while the supplementation of Fe(III) could further enhance Sb natural purification.

Stimulated leaching of metalloids along 3D-printed fractured rock vadose zone

Fractured rock aquifers are susceptible to contamination, with metal(loid)s rapidly migrating from poorly developed overburden to the fractured rock vadose zone and thus into groundwater. Compared to typical porous aquifers, retention effects within the rock matrix are small, and rapid advection along fractures leads to a higher risk of groundwater contamination. However, the highly complex anisotropic pathways of natural fractures hinder research in this field. To construct reproducible fractures, this study used 3D printing following Computed X-ray Microtomography (μCT) scans of a fractured rock collected in a natural limestone aquifer. Stimulated metalloid release was observed in the fractured rock during column leaching, and the leachate concentrations of arsenic (As) and antimony (Sb) increased by up to 17.5 and 36.4 times, respectively, compared with the porous vadose zone. Fluctuations in fracture metalloid release patterns in dissolved and adsorbed phases were attributed to retention and filtration effects induced by soil particles within fractures. Geophysical properties of the porous overburden, especially the aggregation characteristics, greatly affected the non-equilibrium leaching behavior of As, but had a limited effect on the near-equilibrium leaching of Sb, which was explored by modifying the surficial soil layer with either montmorillonite clay or charcoal. The results of this study provide a novel method and useful information for modeling and risk assessment of fractured rock aquifers.

Woodland for Sludge Disposal in Beijing: Sustainable?

The sludge products of urban sewage treatment plants in Beijing are increasing year by year, and there is a large amount of stagnation, which requires scientific and reasonable disposal strategies. Currently, the woodland in the mountainous area of Beijing is considered the main means for sludge disposal; however, because the heavy metals in the sludge may cause potential pollution to the soil and groundwater, it is unclear how much sludge can be applied per unit area. To ensure the sustainable disposal of sludge, it is necessary to measure the risk of heavy metals on soil and groundwater under different sludge application rates to determine the most scientific disposal plan. In this study, the undisturbed soil columns obtained from the field were used to clarify the migration behaviors and accumulation of eight hazardous heavy metals under simulated rainfall conditions, and three sets of tests (the application rates of sludge products were 30 t·ha−1·a−1, 60 t·ha−1·a−1 and 120 t·ha−1·a−1 respectively) were set based on the supply–demand relationship between Beijing’s annual sludge output and the woodland area available for sludge disposal. The results showed that there were significant differences in the migration rules of heavy metals under different application rates, which were mainly reflected in the differences in accumulation in each layer of the soil. In terms of the leaching efficiency of heavy metals, except for Cadmium, the leaching rates of other heavy metals did not exceed 0.1%, indicating that most heavy metals accumulated in the soil. During the application process of sludge products, Arsenic and Cadmium posed a greater potential risk to groundwater than other heavy metals, to which should be paid sufficient attention. Based on the accumulation of heavy metals in soil, Arsenic was the main factor limiting the amount and frequency of sludge product application. The application rate of 60 t·ha−1·a−1 was preferred compared with the other two tests because it presented minimal risk to groundwater and soil in the short term, while the total amount of sludge disposal can be maximized.

Distribution Characteristics and Relevance of Heavy Metals in Soils and Colloids Around a Mining Area in Nanjing, China

Heavy metal pollution in agricultural soils poses a direct threat to food safety and human health. It has been shown that the colloids is the carrier of heavy metal transport in the polluted soil by heavy metals, but the sources of heavy metals in the soil and colloids and their interrelations are not transparent at present. This study aims to investigate the distribution characteristics of heavy metals in agricultural soils near mining areas, and reveal the relevance of heavy metal content in colloids with total content in soils and their chemical species in soils. Results showed that the concentrations of Mn, Zn, and Pb in agricultural soils and colloids were higher than those of other heavy metals. The content of heavy metals in colloids was positively correlated with the total content of heavy metals in soil. Heavy metals in soil could be easily combined by humus-like substances and tryptophan-like protein in the colloids. The primary source of heavy metals in soil and colloids was mining activities. This study provides theoretical support for revealing the pollution characteristics and migration of heavy metals in agricultural soils and colloids around mining areas.

Simultaneous decontamination of arsenite and antimonite using an electrochemical CNT filter functionalized with nanoscale goethite

The co-presence of arsenic (As) and antimony (Sb) in water bodies has been commonly reported. The toxicity of As and Sb varies with different speciation. Herein, we designed a dual-functional electrochemical filter toward "one-step" detoxification and sequestration of highly toxic As(III) and Sb(III). The key to this technology is a functional anodic filter consists of nanoscale goethite and carbon nanotubes (CNT). Results showed that 97.9% As(III) and 91.9% Sb(III) transformation and 86.4% As_total and 70.1% Sb_total removal efficiency can be obtained over 2 h continuous filtration under optimized conditions. The As_total removal kinetics and efficiency enhanced with flow rate and applied voltage (e.g., the As_total removal efficiency increased from 62.9% at 0 V to 86.4% at 2.5 V). This enhancement in kinetics and efficiency can be explained by the synergistic effects of the flow-through design, plentiful exposed sorption sites, electrochemical reactivity, and nanoscale goethite. Moreover, the proposed technology works effectively across a wide pH range. Only negligible inhibition was observed in the presence of nitrate, chloride, and carbonate. Exhausted hybrid filters can be effectively regenerated by using chemical wash with NaOH solution. This study not only revealed the different adsorption behaviors of As(III) and Sb(III) on the hybrid filters, but also provided new insights into rational design of continuous-flow filters toward simultaneous decontamination of As(III) and Sb(III).