Viscosity modification enhanced the migration and distribution of colloidal Mg(OH)2 in aquifers contaminated by heavy metals

Pollution characteristics and source apportionment of heavy metal(loid)s in soil and groundwater of a retired industrial park

Heavy metal(loid)s (HMs) pollution has become a common and complex problem in industrial parks due to rapid industrialization and urbanization. Here, soil and groundwater were sampled from a retired industrial park to investigate the pollution characteristics of HMs. Results show that Ni, Pb, Cr, Zn, Cd, and Cu were the typical HMs in the soil. Source analysis with the positive matrix factorization model indicates that HMs in the topsoil stemmed from industrial activities, traffic emission, and natural source, and the groundwater HMs originated from industrial activities, groundwater-soil interaction, groundwater-rock interaction, and atmosphere deposition. The sequential extraction of soil HMs reveals that As and Hg were mainly distributed in the residue fraction, while Ni, Pb, Cr, Zn, Cd, and Cu mainly existed in the mobile fraction. Most HMs either in the total concentration or in the bioavailable fraction preferred to retain in soil as indicated by their high soil-water partitioning coefficients (Kd), and the Kd values were correlated with soil pH, groundwater redox potential, and dissolved oxygen. The relative stable soil-groundwater circumstance and the low active fraction contents limited the vertical migration of soil HMs and their release to groundwater. These findings increase our knowledge about HMs pollution characteristics of traditional industrial parks and provide a protocol for HMs pollution scrutinizing in large zones.

Process, influencing factors, and simulation of the lateral transport of heavy metals in surface runoff in a mining area driven by rainfall: A review

The lateral transport of heavy metals can expand the scope of original contamination, and an accurate prediction of heavy metal migration is necessary to control heavy metal transport. However, previous studies have mainly focused on the migration of soil pollutants in the runoff-soil-groundwater system, whereas research on the lateral migration of heavy metals in surface soil driven by rainfall is relatively scarce. Therefore, in this study we analyzed the horizontal migration of water-soluble heavy metals with surface runoff and non-water-soluble heavy metals with sediment particles, investigated the main factors affecting the processes of runoff and sediment transport and the main factors affecting the mobility of heavy metals in soils, summarized the existing methods for the simulation of heavy metal transportation. The construction of a lateral migration model based on the migration mechanism of soil heavy metals, the hydrological model, and the application of the lateral migration model should be the focus of future research. This study provides a theoretical basis for establishing a model of the lateral migration of soil heavy metals and is of great significance for the prevention and control of the risks related to the lateral migration of soil heavy metals.

Effects of Silicic Acid on Leaching Behavior of Arsenic from Spent Magnesium-Based Adsorbents Containing Arsenite

The spent adsorbents left after treating arsenic-contaminated water contain large amounts of arsenic. These spent adsorbents may come into contact with silicic acid leached from soil or cementitious solidification materials in the disposal environment. Thus, it is important to evaluate the effects of silicic acid on spent adsorbents containing arsenic. In this study, the effects of silicic acid on spent Mg-based adsorbents (magnesium oxide (MgO) and magnesium hydroxide (Mg(OH)2)) containing arsenite were investigated. The arsenic leaching ratios of both spent adsorbents decreased slightly with an increase in the initial silicic acid concentration of the eluent. The arsenic leaching ratio decreased from 1.24% to 0.69% for MgO and from 5.97% to 4.71% for Mg(OH)2 at an initial Si-normalized concentration of 100 mg/L. The primary mechanism behind the inhibition of arsenic leaching by silicic acid was determined to be the difficulty of arsenic desorption due to the coating effect following the adsorption of silicic acid species. The results indicate that the arsenic leaching related to the ion exchange reaction with silicic acid hardly occurred for the spent Mg-based adsorbents. Compared with various spent Mg-based and Ca-based adsorbents, the spent MgO adsorbent exhibited the highest environmental stability and best performance.

The Migration and Deposition Behaviors of Montmorillonite and Kaolinite Particles in a Two-Dimensional Micromodel

The pick-up, migration, deposition, and clogging behaviors of fine particles are ubiquitous in many engineering applications, including contaminant remediation. Deposition and clogging are detrimental to the efficiency of environmental remediation, and their mechanisms are yet to be elucidated. Two-dimensional microfluidic models were developed to simulate the pore structure of porous media with unified particle sizes in this study. Kaolin and bentonite suspensions were introduced to microfluidic chips to observe their particle deposition and clogging behaviors. Interactions between interparticle forces and particle velocity profiles were investigated via computational fluid dynamics and discrete element method simulations. The results showed that (1) only the velocity vector toward the micropillars and drag forces in the reverse direction were prone to deposition; (2) due to the negligible weight of particles, the Stokes number implied that inertia was not the controlling factor causing deposition; and (3) the salinity of the carrying fluid increased the bentonite deposition because of the shrinkage of the diffused electrical double layer and an increase in aggregation force, whereas it had little effect on kaolin deposition.

Evaluation of three common alkaline agents for immobilization of multi-metals in a field-contaminated acidic soil

We investigated three common alkaline agents (NaOH, CaO, and Mg(OH)₂) for immobilization of four heavy metals (Pb, Zn, Cu, and Cd) in a field-contaminated soil and elucidated the underpinning principles. NaOH caused the highest pH spike in the soil, while CaO and Mg(OH)₂ served as a longer-lasting source of OH^-. Amending the soil with CaO or Mg(OH)₂ at ≥0.1 mol as OH^- (kg·soil)^-1 for 24 h was able to immobilize all four metals, while NaOH failed. NaOH leached up to 3 times more organic carbon than CaO and Mg(OH)₂, resulting in elevated leachability of the metals. Column elution tests showed that amendments by CaO and Mg(OH)₂ lowered the leachable Pb²⁺, Zn²⁺, Cu²⁺, and Cd²⁺ by 52-54%, 71-75%, 69-73%, and 68%, respectively, after 1440 pore volumes of elution. Sequential extraction revealed that the soil amendments converted the exchangeable fraction of the metals to the much less available forms. XRD and FTIR analyses indicated that formation of metal oxide precipitates and complexation with soil organic matter were responsible for the metals immobilization. Taken together the chemical cost, technical effectiveness, and environmental impact, CaO is the most suitable alkaline agent for remediation of soil contaminated with heavy metals.

New insights on the enhanced non-hydroxyl radical contribution under copper promoted TiO2/GO for the photodegradation of tetracycline hydrochloride

TiO₂/graphene oxide (GO) as photocatalyst in the photo-degradation of multitudinous pollutants has been extensively studied. But its low photocatalytic efficiency is attributed to the high band gap energy which lead to low light utilization. Cu-TiO₂/GO was synthesized via the impregnation methods to enhance the catalytic performance. The Cu-TiO₂/GO reaction rate constant for photo-degradation of pollutants (tetracycline hydrochloride, TC) was about 1.4 times that of TiO₂/GO. In 90 min, the removal ratio of Cu-TiO₂/GO for TC was 98%, and the maximum degradation ratio occurred at pH 5. After five cycles, the removal ratio of Cu-TiO₂/GO still exceeded 98%. UV-visible adsorption spectra of Cu-TiO₂/GO showed that its band gap was narrower than TiO₂/GO. Electron paramagnetic resonance (EPR) spectra test illustrated the generation rate of •O₂^- and •OH was higher in Cu-TiO₂/GO system than TiO₂/GO and TiO₂ system. The contribution sequence of oxidative species was •O₂^- > holes (h⁺) > •OH in both TiO₂/GO and Cu-TiO₂/GO system. Interestingly, the contribution of •OH in Cu-TiO₂/GO was less than that in TiO₂/GO during the photo-degradation process. This phenomenon was attributed to the better adsorption performance of Cu-TiO₂/GO which could reduce the accessibility of TC to •OH in liquid. The enhanced non‑hydroxyl radical contribution could be attributed to that the more other active species or sites on (nearby) the surface of Cu-TiO₂/GO generated after doping Cu. These results provide a new perspective for the tradition metal-doped conventional catalysts to enhance the removal of organic pollutants in the environment.