Reactive transport of Cd(II) and Cr(VI) in natural porous media: Influencing factors and mechanism.

Reactive transport and retention of Cd, Pb, and Zn under coexisting multimetal-xanthate conditions in porous media.

Modeling and visualizing the transport and retention of cationic and oxyanionic metals (Cd and Cr) in saturated soil under various hydrochemical and hydrodynamic conditions

Microplastics （MPs） are a type of emerging contaminants that pose a potential threat to global terrestrial ecosystems. The accumulation of MPs in soil inevitably affects soil physical and chemical properties， both directly and indirectly. Additionally， owing to their small size and surface features， MPs have excellent sorption capacity for both organic and inorganic materials， thus affecting their fate in the environment. However， the influence of MPs on heavy metal sorption and transport in soil is still not fully understood. In this study， polyethylene （PE） and Cd were selected as research objects， and on the basis of clarifying the adsorption mechanism of Cd on PE MPs， the effects of PE concentration and particle size on Cd release and transport behavior in soil under different ionic strengths and types （Ca2+ and Na+） were studied using column leaching experiments. The results of the batch experiments showed that the adsorption capacity of PE MPs for Cd2+ decreased with the increase in particle size. Scanning electron microscopy （SEM）， Fourier transform infrared spectroscopy （FTIR）， and Zeta potential were used to analyze the properties of PE MPs and adsorption behavior of Cd2+ onto MPs. The adsorption was mainly a physical process and was controlled by intra-particle diffusion. The adsorption kinetics could be described well by the quasi-second-order kinetics and Webber-Morris model. The adsorption isotherm conformed to the Langmuir model， indicating monolayer adsorption. The results of leaching experiments showed that the effect of PE MPs on Cd release and transport in soil was related to the CaCl2 concentration. At high ionic strength （0.05 mol·L-1 and 0.1 mol·L-1）， PE promoted the transport of Cd. The effluent concentration of Cd2+ increased from 6.48 mg·L-1 and 16.79 mg·L-1 to 7.12 mg·L-1 and 23.45 mg·L-1， whereas at low ionic strength （0.01 mol·L-1）， Cd transport was inhibited by PE MPs， and the effluent concentration of Cd2+ decreased from 0.66 mg·L-1 to 0.57 mg·L-1. The larger the amount of PE added， the more significant the promoting or inhibiting effect. Additionally， the release and transport of Cd in soil were also affected by the MPs particle size and concentration. When the addition amount was small （1%， 4%）， the large-sized MPs were more conducive to the transport of Cd in soil. When the addition amount was large （7%， 20%）， MPs with small particle sizes promoted Cd2+ transport more significantly. When the leaching solution used was NaCl， soil permeability decreased significantly. PE MPs had no significant effect on Cd release and transport but changed the stability of soil aggregates. In conclusion， PE MPs could change the release and transport behavior of Cd in soil， and the impact results were not only related to the particle size and content of MPs but were also influenced by the chemical properties of the soil solution.

Impact of chemical aging on pyrogenic carbon colloid facilitated transport and transformation of Cr (VI) in anoxic environments

Microplastics/nanoplastics in porous media: Key factors controlling their transport and retention behaviors

A column study was conducted to determine the effect of city compost, lime, gypsum, and phosphate on cadmium (Cd) mobility in three well‐recognized benchmark soils of India [viz., (Islamnagar) Vertisol, (Amarpur) Inceptisol, and (Khala) Alfisol]. Columns made of PVC were filled with soil treated with different treatment doses [viz., 0.5% city compost, 1% city compost, 2% city compost, 2.5 t lime/ha, 5 t lime/ha, 2.5 t lime/ha+0.5% city compost, 2.5 t gypsum/ha, 2.5 t gypsum/ha+0.5% city compost, and 100 kg P2O5/ha as potassium phosphate (KH2PO4). The columns were leached with 100 mg L−1 Cd under saturated condition. The amount of water moving through the soils was measured as the pore volume. A delayed breakthrough curve (BTC) of Cd in the presence of lime has been observed in all the studied benchmark soil series. Among the treatments, lime application reduced the movement of Cd from surface soil to lower depth of soil to a large extent resulted in 9, 25, and 45% more retention of Cd in surface soil of the Islamnagar, Amarpur, and Khala series respectively. Explanation for reduced Cd mobility in limed soil can be derived from pH changes of soils. In comparison to control soil, phosphate application caused 6, 21, and 30% more retention of Cd in surface soil in the Islamnagar, Amarpur, and Khala series, respectively. Combined application of lime and city compost reduced the movement of Cd in the soil profile. It appears that organic matter controls the sorption of Cd in soils. The amount of Cd sorbed increased with increasing organic carbon content, but gypsum application may leach Cd beyond the root‐zone depth. A rapid breakthrough curve was observed under gypsum‐treated soils. Retardation factor revealed that a somewhat lower degree of Cd retention occurred in the Khala series, which might possibly be attributed to less clay content and low pH. Overall, the column study indicated that total Cd accumulation occurred up to depths of 5–7.5 cm, 7.5–10 cm, and 10–15 cm in soils of Islamnagar, Amarpur, and Khala series, respectively.

The transport of metals to groundwater below hazardous waste sites is of great environmental concern. The movement of a particular metal is determined by (1) the soil's chemical and physical properties, (2) the amount and form of the metal, and (3) the composition of the soil or waste solution with which the metal is associated. Field and laboratory studies were conducted to examine these factors affecting metal mobility in subsoils. The objectives of the field studies were to determine the distribution and chemical forms of metal contaminants in subsoil at sites with known or suspected potential for groundwater contamination. The objectives of the laboratory studies were to determine the effect of soil properties, solute concentration, solution composition, and redox status on the retention and transport of Cd and Pb in subsoil materials. In the field and laboratory studies, the soil properties controlling Pb retention and mobility were pH, CEC, metal oxides (Mn oxides, amorphous Fe oxides, and free Fe oxides), and soil particle-size distribution. Retention of Cd was controlled by pH, CEC, amorphous Fe oxides, and particle-size distribution. Copper mobility was controlled by the three metal oxides. The Fe oxides (amorphous and crystalline) controlled Ni and Zn retention. Arsenic retention was controlled by pH, CEC, and the metal oxides. Redox potential was also important in Cd and Pb retention, with greater metal retention under oxidizing conditions. In the laboratory studies, higher concentrations of Cd and Pb resulted in lower proportions of metals being retained by the soil. Therefore, the greater the concentration of the metal, the faster it will move through the soil. The retention of Pb was greater than Cd. The retention of Cd and Pb was greater in a dilute salt solution (0.005M Ca(NO$\sb3)\sb2)$ and in a simulated oil field waste. Retention was less in a synthetic municipal landfill leachate and simulated acidic metals waste. Organic complexation of Cd and Pb reduced their retention and thus increased in their mobility. The low pH of the acidic metals waste and competition from other metals reduced Cd and Pb retention in this waste. It was concluded that the composition of the waste solution may override soil factors in controlling trace metal mobility.

Modeling Cadmium Transport in Neutral and Alkaline Soil Columns at Various Depths

Insight into the mechanism of phosphate and cadmium co-transport in natural soils

The fate of materials undergoing transport and reactions in natural porous media sometimes depends on the time of exposure of the conveyed material to other materials present in the system. The distribution of groundwater age, the effects of mineral chemical heterogeneity on reactive solute transport, and the occurrence of lag in reaction systems are some areas of hydrogeology that involve exposure time in an important way. A general balance equation for accounting for such effects is provided through an extended transport operator that incorporates generalized exposure time as an additional independent coordinate. Evolution of material distributions over exposure time appears within this transport operator as a convective process that represents space‐ and time‐dependent generalized exposure of material constituents undergoing physical transport and nonequilibrium chemical and microbiological mass transformations. The general equation is derived from basic mass balance arguments by treating the constituents as a mixture of overlapping continua and developing evolution equations for the mixture material densities in the new dimensions of space, time, and exposure time. Example applications of the approach to each of the three examples above are described.

Processes controlling metal transport and retention as metal-contaminated groundwaters efflux through estuarine sediments

Geochemical factors controlling the mobilization of geogenic cadmium in soils developed on carbonate bedrocks in Southwest China

Nonequilibrium surface complexation reactions have been found to substantially affect U(VI) transport in natural porous media both in laboratory and field scale experiments. Nonequilibrium sorption behavior occurs on multiple time scales and is a result of diffusion-limited transport in immobile intra-grain and intra-aggregate pore water. Experimental data on U(VI) transport was successfully described with a recently developed reactive transport model that accounted for the nonequilibrium adsorption processes through the formulation of a multi-rate surface complexation model treating surface complexation as kinetic reactions. In the present work, a benchmark problem set has been developed for testing existing or newly developed reactive transport codes on their capability to simulate multi-rate surface complexation and dual-domain multi-component reactive transport of U(VI). The benchmark problem consists of three individual component problems on the basis of previous studies investigating the desorption of U(VI) from radionuclide-contaminated sediment from the Hanford 300A site, Washington, USA. Starting with a single-domain model considering constant hydrochemical conditions (component problem 1), the complexity of the model was stepwise increased. In the component problem 2 dual-domain first-order mass transfer was added. The principal problem also included dual-domain mass-transfer, but was further extended for changing hydrochemical conditions in the column’s inflow water, which resulted in drastic changes in the U(VI) desorption pattern due to surface complexation reactions. For the three individual component problems, the corresponding simulation results agree very well among four well-known and thoroughly tested independent reactive transport codes, indicating that the proposed benchmark problem set is a suitable test case.

Purpose The contamination of agricultural soils by heavy metals is a worldwide problem. Organic amendments can be used for the immobilization and binding of heavy metal ions in soils by complexation, adsorption, and precipitation. A field trial was carried out to evaluate the influence of some low-cost organic materials such as rice straw (RS), green manure (GM), and pig manure (PM) on the distribution of Cu and Cd and the retention of these metals by organic matter fractions in heavy metal-polluted soils. Materials and methods The experiment was conducted in Miaoyunao Village, Daye County, Hubei province, China. PM, GM (peanut plants), and RS were obtained from a farm close to the village. Sixteen treatments with three replicates were designed. Soil chemical properties such as soil pH, electrical conductivity (EC), organic matter (OM), and available P were measured by standard methods. Soluble/ exchangeable, organic-bound, inorganic precipitates and residual Cu and Cd in the soil were sequentially extracted and analyzed. The amounts of Cu and Cd bound with soil particulate organic matter (POM) fractions and humic substances were also determined. Results and discussion The addition of organic amendments declined significantly the concentrations of soluble/ exchangeable Cu and Cd, but increased the amounts of these metals in organic-bound and inorganic precipitate forms in the soil. RS was more effective than GM and PM in diminishing the solubility of Cu and Cd. The largest retention for Cu and Cd by humic substances and POM was noticedinRStreatments,whereasthelowestwasfoundinPM treatments. Humic substances showed higher potential in the fixation of Cu and Cd than POM fractions. The conversion of soluble/exchangeable Cu and Cd to other insoluble forms after the application of organic amendments may be ascribed to the increases of soil OM, pH, EC, and available P contents. The highest binding of Cu and Cd with POM fractions and humic substances after the incorporation of RS mainly resulted from the greatest increase of soil OM contents. Conclusions RS, GM, and PM can be employed as good and cheap substances for the immobilization of Cu and Cd in heavy metal-polluted soils. RS was the best amendment in decreasing the solubility of Cu and Cd, and also in enhancing the retention of these metals by humic substances and POM fractions in the soil. Futures studies should focus on the influence of these organic amendments or their mixtures on the phytotoxicity of Cu and Cd for different plants in heavy metal-contaminated soils.

Cadmium (Cd) can be leached from soil into the groundwater and exhibit its adverse effect on the health of animals and humans. While previous studies have studied the process of Cd transport in water-saturated sand columns, literature regarding Cd transport in soil is scarce. The aim of this experiment was to investigate the transport of Cd in soil columns and biochar application rate effects on the mobility and distribution of Cd in soil. The red paddy soil was collected from the paddy of Changsha County, Hunan Province in southern China. Batch sorption and column experiments were conducted to study the adsorption isotherms of Cd2+ and its mobility at different biochar application rate treatments (0, 0.5, 1, 1.5, and 2%) referenced here as A0, A10, A20, A30, and A40, respectively. The Cd concentration of in effluent samples and digestion solutions was measured by inductively coupled plasma optical emission spectrometer (ICP-OES, Thermo Fisher Scientific, USA). After finishing the column experiment, columns were dissected into five layers (1-cm segments), the Cd fractions in soil were performed by the European Community Bureau of Reference (BCR). The amount of Cd sorption among treatments decreased in the order of A40 > A30 > A20 > A10 > A0, and the Langmuir model was more suitable to study the Cd2+ adsorption on biochar-amended soil than Freundlich model. Breakthrough curves showed that increasing biochar application rate increased the initial breakthrough time, whereas the pore-water velocity and dispersion coefficient were 81.0 and 99.8% lower in the A40 treatments than in the A0 treatments, respectively. Increasing biochar application rate enhanced the pH but reduced redox potential (Eh) in the most of effluents. Compared with A0, the concentration of Cd retained in soil columns increased by 86.6% in the A40 treatments. However, BCR sequential extractions showed that biochar addition in A40 treatments increased the acid soluble fraction but reduced the reducible fraction. In A40 treatments, compared with the 0-1-cm soil layer, the relative Cd concentration (N/Ni) in the 1-2-, 2-3-, 3-4-, and 4-5-cm soil layers increased by 5.4, 10.9, 14.3, and 21.9%, respectively. Biochar application in A40 treatments showed strong capacity for retarding Cd transport in soil, while the potential mobility of Cd in soil should be considered.