Measurement of contaminant adsorption on soils using cycling modified column tests

Transport of dimethyl phthalate on loess with modified bentonite: A batch and column test investigation

Phthalic acid ester (PAE) is a toxic pollutant commonly found in high concentrations in municipal solid waste landfills. Soil-bentonite is widely used as a barrier material to control groundwater contaminants from landfill leachates. Traditional soil-bentonite materials always have a limited capacity for organic pollutant adsorption. To address this issue, the adsorption and transport behavior of dimethyl phthalate (DMP) on loess amended with two kinds of modified bentonite (HTMAC-B, modified with hexadecyltrimethylammonium chloride; CMC-B, modified with hydrophobic cationic surfactant, and carboxymethyl cellulose) were investigated. The kinetics of DMP adsorption indicates that film diffusion contributes significantly to the kinetic adsorption of DMP on HTMAC-B. The adsorption isotherm results showed that partitioning dominated DMP adsorption on loess with both modified bentonites. Owing to the in-ionic sites in HTMAC-B, which attracted hydrophobic compounds such as DMP, the adsorption capacity of 5 % HTMAC-B-amended loess (LH) was increased by a factor of 3.2. However, because CMC-B provided mostly ionic sites, 5 % CMC-B-amended loess (LC) had a little effect on DMP adsorption. The hydraulic conductivity values of LH and LC were 5.95 × 10^-10 and 1.65 × 10^-11 m/s, respectively. The X-CT result showed that there is a significant porosity change for both LH and LC. Dual-porosity model reveals that the leaching process primarily affects micro-pores, rather than larger pores in the soil matrix. The predicted retardation factors for LH and LC were 38.89 and 9.67, respectively. When using loess-bentonite as barrier material, the amendment of HTMAC-B and CMC-B can help to increase the retardation ability and reduce the permeability, respectively.

Bisphenol A adsorption and transport in loess and cationic surfactant/hydrophilic polymer modified bentonite liners

Bisphenol A (BPA) is a toxic endocrine disruptor often found in landfill leachate. Adsorption behaviors and mechanisms of BPA onto organo-bentonites amended loess, e.g., Hexadecyltrimethylammonium chloride-bentonite (HTMAC-B) and Carboxymethylcellulose-bentonite (CMC-B) were experimentally investigated. The adsorption capacity of loess amended by HTMAC-B (LHB) and CMC-B (LCB) is 4.2 and 4 times greater than that of loess (L), respectively. It is attributed to the increase of hydrogen bonds and hydrophobic lateral interactions between the adsorbent and the adsorbate. The binary (Pb²⁺-BPA) systems may enhance BPA adsorption onto the samples by the formation of coordination bonds between the hydroxyl group of BPA and Pb²⁺ ions. A cycled column test was used for investigating the transport behavior of BPA in LHB and LCB samples. The hydraulic conductivity of loess amended by the organo-bentonite (e.g., HTMAC-B, CMC-B) is generally lower than 1 × 10^-9 m/s. Especially for CMC-B amended loess, the hydraulic conductivity can be reduced to 1 × 10^-12 m/s. This guarantees the hydraulic performance of the liner system. Transport behavior of BPA in cycled column test is explained by the mobile-immobile model (MIM). Modelling results showed that loess amended by organo-bentonites can increase the breakthrough time of BPA. In comparison to loess-based liner, the breakthrough time of BPA for LHB and LCB can be increased by a factor of 10.4 and 7.5, respectively. These results indicate that organo-bentonites can serve as a potentially effective amendment for improving the adsorption of loess-based liners.

Investigating immobilization efficiency of Pb in solution and loess soil using bio-inspired carbonate precipitation

Lead (Pb) metal accumulation in surrounding environments can cause serious threats to human health, causing liver and kidney function damage. This work explored the potential of applying the MICP technology to remediate Pb-rich water bodies and Pb-contaminated loess soil sites. In the test tube experiments, the Pb immobilization efficiency of above 85% is attained through PbCO₃ and Pb(CO₃)₂(OH)₂ precipitation. Notwithstanding that, in the loess soil column tests, the Pb immobilization efficiency decreases with the increase in depth and could be as low as approximately 40% in the deep ground. PbCO₃ and Pb(CO₃)₂(OH)₂ precipitation has not been detected as the majority of Pb²⁺ combines with -OH (hydroxyl group) when subjected to 500 mg/kg Pb²⁺. The alkaline front promotes the chemisorption of Pb²⁺ with CO₃^2- reducing the depletion of quartz mineral close to the surface. However, OH^- is in shortage in the deep ground retarding the Pb immobilization. The Pb immobilization efficiency thus decreases with the increase in depth. Quartz and albite minerals, when subjected to 16,000 mg/kg Pb²⁺, appear not to intervene in the chemisorption with Pb²⁺ where the chemisorption of Pb²⁺ with CO₃^2- plays a major role in the Pb immobilization. Compared to the nanoscale urease applied to the enzyme-induced carbonate precipitation (EICP) technology, the micrometer scale ureolytic bacteria penetrate into the deep ground with difficulty. The 'size' issue remains to be addressed in near future.

Adsorption of flupyradifurone onto soils: kinetics, isotherms, and influencing factors

The study of the adsorption properties of pesticides in soil is essential to assessing the risk of their pollution of nearby aquatic environments. To reveal the adsorption mechanisms of flupyradifurone (FPO) on soil, batch experiments in five different soils were carried out in this study. The adsorption kinetics and isotherms of FPO in five soils were well fitted by using several models (R² = 0.922-0.998). It was found that both physical and chemical adsorption were included in the adsorption process of FPO in soils; the monolayer adsorption of FPO occurred with a non-uniform energy distribution on the soil surface, and the internal particle diffusion was not the only rate-controlling step. The adsorption coefficients calculated by using the Langmuir (K_L) and Freundlich (K_F) models were 0.0158-0.0982 and 1.053-9.798, respectively. In addition, the main factors affecting the adsorption of FPO in soil were investigated by stepwise regression fitted with the adsorption coefficient (K_d) and the soil properties. It was found that the organic carbon content was the main factor (R² = 0.857, p < 0.05). Therefore, the organic carbon adsorption coefficients (K_oc) were calculated. The results (1.0532-5.6529) indicated that FPO has a low affinity and high mobility in the soils, and may cause water environment pollution around the soil. Therefore, FPO should be used cautiously in paddy fields. These research findings were important for elucidating the sorption behaviour and transport of FPO in soil.