The latest research trends in the removal of cesium from radioactive wastewater: A review based on data-driven and visual analysis.

The widespread adoption of nuclear energy has increased the amount of radioactive cesium (Cs) that is discharged into waste streams, which can have environmental risks. In this paper, we provide a comprehensive summary of current advances in aqueous Cs removal by employing a bibliometric analysis. We collected 1580 articles related to aqueous Cs treatment that were published on the Web of Science database between 2012 and 2022. By applying bibliometric analysis combined with network analysis, we revealed the research distribution, knowledge base, research hotspots, and cutting-edge technologies in the field of aqueous Cs removal. Our findings indicate that China, Japan, and South Korea are the most productive countries with respect to Cs removal research. In addition, both historic events and environmental threats might have contributed to research in Asian countries having a higher focus on Cs removal as well as strong international cooperation between Asian countries. A detailed keyword analysis reveals the main knowledge base for aqueous Cs removal and highlights the potential of the adsorption-based method for treating Cs contamination. Furthermore, the results reveal that exploration of functional materials is a popular research topic in the field of Cs removal. Since 2012, novel materials, including Prussian blue, graphene oxide, hydrogel and nanocomposites, have been widely investigated because of their high capacity for Cs removal. On the basis of the detailed information, we report the latest research trends on aqueous Cs removal, and propose future research directions and describe the challenges related to effective Cs treatment. This scientometric review provides insights into current research hotspots and cutting-edge trends in addition to contributing to the development of this crucial research field.

Leaching of heavy metal(loid)s from historical Pb-Zn mining tailing in abandoned tailing deposit: Up-flow column and batch tests.

This study aims to investigate the water-leaching characteristics of heavy metal(loid)s (HMs) from historical Pb-Zn mine tailing of an abandoned tailing deposit in eastern China. Up-flow column and batch leaching tests were conducted at different liquid-to-solid (L/S) ratios to estimate the releases of HMs and investigate the controlling mechanisms. Calcite and silicate were the dominant minerals in the tailing and the HMs contents followed the order of Zn (2371 mg/kg) > Pb (2061 mg/kg) > Cu (109 mg/kg) > Cr (47.8 mg/kg) > As (15.9 mg/kg) > Cd (5.1 mg/kg). Moreover, considerable fractions of Pb, Zn, and Cd existed in the acid-soluble forms (41-47%). Column and batch leaching tests consistently showed that limited quantities (<0.002%) of HMs could be leached from this historical tailing. In particular, variations in column conditions (e.g., length, flow rate, and initial saturation) significantly affected the release fluxes from the columns but had a relatively limited effect on the leaching mechanisms. The estimated results of HM release suggested that the leaching process was predominantly solubility-controlled and the dissolution of Ca-bearing minerals (e.g., calcite) primarily controlled the release of HMs. The studied tailing had a limited impact on the quality of the surrounding aquatic environments because the water-leaching concentrations of HMs were generally lower than the Chinese standards for drinking water. Only for Pb, the leaching results in column tests were significantly lower than those in batch tests; whereas the results in column tests for other HMs were comparable to those in batch tests to a certain extent. Based on the column test results, the amounts of HMs potentially released from the abandoned tailing deposit (height, 10 m; footprint area, 30,000 m2; tailing dry density, 1.9 × 103 kg/m3) followed a decreasing order of Zn (4.2 × 105 kg) > Cu (2.3 × 104 kg) > Pb (1.4 × 104 kg) > Cr (2.3 × 104 kg) > Cd (1.6 × 103 kg) > As (1.2 × 103 kg) over the 75-year assessment period (corresponding to an L/S ratio of 10 L/kg with an annual precipitation of 1500 mm).

N-doped biochar-catalyzed dechlorination of carbon tetrachloride in sulfide-containing aqueous solutions: Performances, mechanisms and pathways.

Nitrogen-doped biochar (N-BC) has been widely concerned in the field of environmental protection. This study verified the alfalfa-based N-BC pyrolyzed at different temperatures is able to catalyze carbon tetrachloride (CT) dechlorination in sulfide-containing aqueous solutions under normal environmental pH range (6.3 ∼ 8.3) effectively, with Cl-, trichloromethane (CF), CS2 and HCO3- as predominated products. Higher pyrolysis temperature resulted in larger specific surface area, more pores and better catalytic capacity. The N-BC had a good tolerance to water matrix in catalyzing CT dechlorination by sulfide, while the higher pH value or higher dosage of sulfide or N-BC was beneficial to catalytic CT dechlorination. The generation of CS2 was the major CT dechlorination pathway, attributing to the SN2 nucleophilic substitution by newborn C-SS- structure generating from the interaction between pyridine-N and sulfide. Besides, generation of CF via hydrogenolysis process was the minor CT dechlorination pathway, owing to the enhanced electron transfer by pyrrole-N, graphitic-N and quinones on surface of N-BC. It was the first time that N-BC was found to be effective in catalyzing the hydrogenolysis process of CT dechlorination. This study emphasized the importance of N-BC in restoring chlorinated hydrocarbons polluted aquatic environment containing sulfide, such as sediments.

Hexachloroethane dechlorination in sulfide-containing aqueous solutions catalyzed by nitrogen-doped carbon materials.

This study demonstrated that nitrogen-doped carbon materials (NCMs) could effectively catalyze the chlorine elimination process in hexachloroethane (HCA) declorination in sulfide-containing environments for the first time. The kobs values of HCA dechlorination by sulfide in the presence of 10 mg/L NCMs were higher than that of no mediator at pH 7.3 by one or two orders of magnitude. The catalytic capabilities of NCMs on HCA dechlorination were evident in common ranges of natural pH (5.3-8.9) and it could be accelerated by the increase of pH but be suppressed by the presence of dissolved humic acid. Moreover, NCMs exhibited much better catalytic capability on HCA dechlorination compared to the carbon materials, mainly owing to the combined contributions of pyridine N, including enhanced nucleophilic attack to HCA molecule by generating newborn C-S-S and activation of HCA molecule by elongating C-Cl bonds. The functions of pyridine N in micron-sized NCMs with mesopores were better than in nano-sized NCMs on HCA dechlorination. These findings displayed the potential of NCMs, when released into sulfide-containing environments, may significantly increase the dechlorination of chlorinated aliphatic hydrocarbons.

Stabilisation/solidification of municipal solid waste incineration fly ash by phosphate-enhanced calcium aluminate cement.

Landfill disposal of municipal solid waste incineration fly ash (MIFA) presents significant environmental and economic burden. This study proposed a novel and high-efficiency approach for stabilisation/solidification (S/S) of MIFA by phosphate-modified calcium aluminate cement (CAC). Experimental results showed that the presence of Pb (the most leachable metal contaminant in the MIFA) retarded the early-stage reaction of CAC, resulting in an extension of setting time and a significant decline of compressive strength of CAC pastes. The incorporation of phosphate additives (10 wt% of binder), especially for trisodium phosphate, in CAC system effectively mitigated the negative impact of Pb on the CAC reaction and reduced the Pb leachability. Elemental mapping results illustrated that Pb2+ coordinated with phosphate to generate insoluble precipitates (e.g., Pb3(PO4)2). The S/S treated MIFA samples fulfilled the compressive strength and leachability requirements for on-site reuse. Overall, this study demonstrated that phosphate-modified CAC is a promising binder for S/S of hazardous MIFA.

Evaluation of the BCR sequential extraction scheme for trace metal fractionation of alkaline municipal solid waste incineration fly ash.

The BCR sequential extraction scheme (SES), initially developed for soils and sediments, is frequently adopted to evaluate the environmental risks of municipal solid waste incineration (MSWI) fly ash. Within the procedure, metals are liberated from the matrix hosting them relying on the selectivity of the chosen chemical reagents or operation conditions. However, the effect of the high content of alkaline substances in MSWI fly ash on the selectivity of acetic acid to acid-soluble fraction metals was ignored. In this study, the feasibility of the BCR SES for evaluating MSWI fly ash was assessed by adjusting the acetic acid washing times in the acid-soluble extraction step. The metal fractionation, as well as mineralogy, morphology, and surface chemistry of the residues after three successive acid washing processes, were analyzed. The results reveal that only easily soluble salts, but not hydroxides, are entirely extracted after the first acid washing (pH∼12.0). Importantly, carbonates (generally reported as an indicator of the complete release of acid-soluble metals) are mostly decomposed only after the third acid washing (pH∼3.8). The incomplete dissolution of calcium carbonates in a single-step acid washing may convey misleading results of metal fractionation and underestimates the environmental risk of potentially toxic elements. Therefore, complete removal of carbonates should be employed as the endpoint of the acid-soluble fraction extraction step in the evaluation of MSWI fly ash. This work can help in selecting proper strategies for fly ash management and developing proper sequential extraction schemes for similar high-alkalinity hazardous waste risk assessment.

Investigation of controlling factors on toxic metal leaching behavior in municipal solid wastes incineration fly ash.

Municipal solid wastes incineration (MSWI) fly ash has drawn worldwide attention for its substantial annual generation capacity and high toxic metals leachability. Although many factors have been shown to affect the leachability of metals in fly ash, the controlling factors, which guide the selection of appropriate risk reduction method, remain unclear. The purpose of this study was to evaluate the effects of the two most important factors, total metal content, and remaining alkaline substances of MSWI fly ash, on the leaching behavior of toxic metals. In this work, a series of leaching tests and sequential extraction procedures were performed for seven fly ash samples collected from one MSWI plant. Results show that particulate size distribution, morphology, and mineralogy of all samples are similar, indicating the effects of these properties on metal leaching behavior can be ignored. In leaching tests, although the leaching behavior in terms of metal species and concentration levels vary as expected, only the leachate Pb concentration in four samples (up to 17.32 mg/L) exceeds the threshold in Chinese regulation (0.25 mg/L). The variation of the leachate Pb concentration is not consistent with the change of the total Pb concentration in fly ash. Further correlation analysis evidences that the acid-soluble Pb, which is highly correlated to the calcium content of fly ash, dominates the concentration of leachate Pb. Notably, when the addition of lime is about 1.5 times over the theoretical value, the concentration of leachate Pb would exceed the threshold regardless of the total Pb concentration in fly ash. Overall, this study demonstrates that the remaining alkaline substances (mainly calcium-bearing compounds), rather than the total content of metals, are the controlling factor of metal leaching behavior in fly ash. Thus, strategies to delicately optimize the quantity of lime addition in acid gas purification process should be considered to minimize MSWI fly ash environmental risks in the future.

Electro-migration of heavy metals in an aged electroplating contaminated soil affected by the coexisting hexavalent chromium.

Cr(VI) was often reported to oxidize soil organic matter at acidic environments due to its high ORP, probably thus changing cationic metal species bound to soil organic matter, and influencing their electro-migration patterns. However, such an effect on the electro-migration was not confirmed in most previous studies. Therefore, this study applied a fixed voltage direct current field on an aged electroplating contaminated clayed soil, with a special interest in the direct or indirect influence of Cr(VI) on the electro-migration of other coexisting metals. After 353 h electrokinetic process, 81% of Zn, 53% of Ni and 22% of Cu in the original soil were electro-migrated into the electrolyte, and most of the remaining concentrated near the cathode. The Cr(VI) oxidized some soil organic matter along its migration pathway, with a pronounced reaction occurred near the anode at low pHs. The resulting Cr(III) reversed its original movement, and migrated towards the cathode, leading to the occurrence of a second Cr concentration peak in the soil. Metal species analyses showed that the amount of metals bound to soil organic matter significantly decreased, while a substantial increase in the Cr species bound to Fe/Mn (hydro-)oxides was observed, suggesting an enhancement of cationic metal electro-migration by the reduction of Cr(VI) into Cr(III). However, the Cr(VI) may form some stable lead chromate precipitates, and in turn demobilize Pb in the soil, as the results showed a low Pb removal and an increase in its acid-extractable and residual fractions after electrokinetic remediation.

Enhancement of phenanthrene adsorption on a clayey soil and clay minerals by coexisting lead or cadmium.

Phenanthrene is commonly present together with heavy metals at many contaminated sites. This study investigated the influence of coexisting lead (Pb(2+)) or cadmium (Cd(2+)) on phenanthrene adsorption on soils. Batch experiments were conducted under different geochemical conditions including pH, mineral structure, organic matter content, and varying amounts of heavy metals. The results showed that the presence of heavy metals in solution at a fixed pH of 5.8±0.1 enhanced phenanthrene adsorption, the extent of which was closely related to the concentrations and the electro-negativity of the metals. The enhancement on phenanthrene adsorption was positively correlated to the amount of adsorbed metals. Although Cd(2+) is a softer Lewis acid, Pb(2+) displayed a more significant effect as it was adsorbed to a greater extent on the soil surfaces. Thus, density of cation accumulation appears to be more influential than metal softness in enhancing phenanthrene adsorption. Moreover, with a portion of organic matter removed by heating at 550°C, there was a stronger enhancement of phenanthrene adsorption by coexisting Pb(2+), indicating an increasingly dominant mechanisms associated with Pb(2+) at a lower organic matter content. Similar enhancement phenomenon was observed on bentonite and kaolinite, probably resulting from the cation-π bonding between the adsorbed soft metal cations and the aromatic ring of phenanthrene in solution. The desorption experiments further suggested that the bonding of phenanthrene adsorption was strengthened in the presence of Pb(2+) and that a larger proportion of adsorbed phenanthrene remained on the soils (residual fraction) even after sequential methanol extractions. Further spectroscopic analyses and surface characterization are required to provide direct evidence of the formation and relative significance of cation-π bond for phenanthrene adsorption.

Influence of soil washing with a chelator on subsequent chemical immobilization of heavy metals in a contaminated soil.

To assess the influence of soil washing with a chelator on the chemical immobilization of heavy metals, batch experiments were performed on the fine fraction of a contaminated soil under various operating conditions. Results show that pre-washing with EDTA facilitated the chemical immobilization of Cu and Cr, while an opposite effect for Pb and Zn was observed, in particular when Ca(OH)(2) was added as the immobilizing agent. Metal fraction analyses of the soils indicate that soil washing can reduce the metal mobility by removing the labile fractions, while it may also destabilize some strongly bound fractions, reversely increasing the mobility and thus compromising the subsequent immobilization performance to some extents. To secure an effective combination of soil washing and chemical immobilization for the remediation of heavy metal-contaminated sites, a comprehensive study on metal fraction distribution in the soil is needed.