The longevity evaluation of multi-metal stabilization by MgO in Pb/Zn smelter-contaminated soils.

The re-mobilization risks of potentially toxic elements (PTEs) during stabilization deserve to be considered. In this study, artificial simulation evaluation methods based on the environmental stress of freeze-thaw (F-T), acidification and variable pH were conducted to assess the long-term effectiveness of PTEs stabilized by MgO in Pb/Zn smelter contaminated soils. Among common stabilizing materials, MgO was considered as the best remediation material, since PTEs bioavailability reduced by 55.48% for As, 19.58% for Cd, 10.57% for Cu, and 26.33% for Mn, respectively. The stabilization effects of PTEs by MgO were best at the dosage of 5 wt%, but these studied PTEs would re-mobilize after 30 times F-T cycles. Acid and base buffering capacity results indicated that the basicity of contaminated soils with MgO treatment reduced under F-T action, and the leached PTEs concentrations would exceed the safety limits of surface water quality standard in China (GB3838-2002) after acidification of 2325 years. No significant changes were found in the pH-dependent patterns of PTEs before and after F-T cycles. However, after F-T cycles, the leaching concentrations of PTEs increased due to the destruction of soil microstructure and the functionality of hydration products formed by MgO, as indicated by scanning electron microscopy (SEM) coupled with energydispersive Xray spectroscopy (EDS) results. Hence, these findings would provide beneficial references for soil remediation assessments of contaminated soils under multi-environmental stress.

The occurrence of "yellowing" phenomenon and its main driving factors after the remediation of chromium (Cr)-contaminated soils: A literature review.

Chromium (Cr) is a highly toxic element, which is widely present in environment due to industrial activities. One of most applicable technique to clean up Cr pollution is chemical reduction. However, the Cr(VI) concentration in soil increases again after remediation, and meanwhile the yellow soil would appear, which is commonly called as "yellowing" phenomenon. To date, the reason behind the phenomenon has been disputed for decades. This study aimed to introduce the possible "yellowing" mechanism and the influencing factors based on the extensive literature review. In this work, the concept of "yellowing" phenomenon was explained, and the most potential reasons include the reoxidation of manganese (Mn) oxides and mass transfer were summarized. Based on the reported finding and results, the large area of "yellowing" is likely to be caused by the re-migration of Cr(VI), since it could not sufficiently contact with the reductant under the effects of the mass transfer. In addition, other driving factors also control the occurrence of "yellowing" phenomenon. This review provides valuable reference for the academic peers participating in the Cr-contaminated sites remediation.

The mechanistic insights into the leaching behaviors of potentially toxic elements from the indigenous zinc smelting slags under the slag dumping site scenario.

Since lager quantities of the zinc (Zn) smelting slags were traditionally dumped at the indigenous Zn smelting sites, the release characterization of potentially toxic elements (PTEs) from the Zn smelting slags under various environmental conditions were of great significance for an environmental risk analysis. The acidification of the Zn smelting slags to pH= 4 and 6 would result in the leaching concentrations of Cd and Mn exceeding the fourth-class standard of surface water quality standard in China (GB3838-2002). Notably, most metals exhibited an amphoteric leaching pattern, where the highest leached concentrations of As, Cd, Cu, Mn, Pb, and Zn were 4.15, 4.21, 140.0, 78.1, 156.9 and 477.0 mg/L, respectively. In addition, the highest release of toxic metals within 96 h reached 0.17 % of As, 3.50 % of Cd, 2.77 % of Cu, 6.92 % of Mn, 0.13 % of Pb, and 2.57 % of Zn, respectively. The combined results of various characterization techniques suggested that the PTEs remobilization effected by rhizosphere-like organic acids were mainly controlled by the precipitation of newly formed Fe, Mn and Al (hydr) oxides and the complexation of organic ligands. The present study results could provide valuable insights into the long-term leaching behaviors of PTEs from the Zn smelting slags to reduce ecological hazard.

Insight into the Adsorption Behaviors of Antimony onto Soils Using Multidisciplinary Characterization.

Antimony (Sb) pollution in soils is an important environmental problem, and it is imperative to investigate the migration and transformation behavior of Sb in soils. The adsorption behaviors and interaction mechanisms of Sb in soils were studied using integrated characterization techniques and the batch equilibrium method. The results indicated that the adsorption kinetics and isotherms of Sb onto soils were well fitted by the first-order kinetic, Langmuir, and Freundlich models, respectively, while the maximum adsorbed amounts of Sb (III) in soil 1 and soil 2 were 1314.46 mg/kg and 1359.25 mg/kg, respectively, and those of Sb (V) in soil 1 and soil 2 were 415.65 mg/kg and 535.97 mg/kg, respectively. In addition, pH ranging from 4 to 10 had little effect on the adsorption behavior of Sb. Moreover, it was found that Sb was mainly present in the residue fractions, indicating that Sb had high geochemical stability in soils. SEM analysis indicated that the distribution positions of Sb were highly coincident with Ca, which was mainly due to the existence of calcium oxides, such as calcium carbonate and calcium hydroxide, that affected Sb adsorption, and further resulted in Sb and Ca bearing co-precipitation. XPS analysis revealed the valence state transformation of Sb (III) and Sb (V), suggesting that Fe/Mn oxides and reactive oxygen species (ROS) served as oxidant or reductant to promote the occurrence of the Sb redox reaction. Sb was mobile and leachable in soils and posed a significant threat to surface soils, organisms, and groundwater. This work provides a fundamental understanding of Sb adsorption onto soils, as well as a theoretical guide for studies on the adsorption and migration behavior of Sb in soils.

A typical case study from smelter-contaminated soil: new insights into the environmental availability of heavy metals using an integrated mineralogy characterization.

Mineralogy was an important driver for the environmental release of heavy metals. Therefore, the present work was conducted by coupling mineral liberation analyzer (MLA) with complementary geochemical tests to evaluate the geochemical behaviors and their potential environmental risks of heavy metals in the smelter contaminated soil. MLA analysis showed that the soil contained 34.0% of quartz, 17.15% of biotite, 1.36% of metal sulfides, 19.48% of metal oxides, and 0.04% of gypsum. Moreover, As, Pb, and Zn were primarily hosted by arsenopyrite (29.29%), galena (88.41%), and limonite (24.15%), respectively. The integrated geochemical results indicated that among the studied metals, Cd, Cu, Mn, Pb, and Zn were found to be more bioavailable, bioaccessible, and mobile. Based on the combined mineralogical and geochemical results, the environmental release of smelter-driven elements such as Cd, Cu, Mn, Pb, and Zn were mainly controlled by the acidic dissolution of minerals with neutralizing potential, the reductive dissolution of Fe/Mn oxides, and the partial oxidation of metal sulfide minerals. The present study results have confirmed the great importance of mineralogy analysis and geochemical approaches to explain the contribution of smelting activities to soil pollution risks.

The geochemical behaviors of potentially toxic elements in a typical lead/zinc (Pb/Zn) smelter contaminated soil with quantitative mineralogical assessments.

This study comprehensively investigated the potential roles of soil mineralogy identified by the automated mineral liberation analysers (MLA) in the prediction of geochemical behavior of toxic metals in the smelter polluted soils. The results from modal mineralogy revealed that the non-reactive silicate phases such as quartz (42.05%) and biotite (40.43%) were the major mineralogical phases. The element deportment showed that fayalite, lead oxide, apatite, galena and wollastonite were identified as the dominant As, Cd, Pb and Zn bearing minerals. Furthermore, MLA analysis also confirmed that Pb was most concentrated in the smaller particles of lead oxide, which significantly enhanced Pb release in reaction with the chemical extractant during chemical kinetic tests. The results from pH-dependent leaching tests indicated that the leaching concentrations of As, Pb and Zn increased at low and high pH values, but were lowest at the neutral pH range. In addition, the results from the kinetic study demonstrated that the second order model provided the best description for the release patterns of the main metal contaminants in the bioavailability and bioaccessibility tests. The integrated geochemical analysis demonstrated that among these studied elements, As showed a typical geochemical pattern, which was predominantly controlled by 90.09% of fayalite. The above study results would have significant implications for soil remediation and risk management of smelter contaminated sites.

The mechanistic understanding of potential bioaccessibility of toxic heavy metals in the indigenous zinc smelting slags with multidisciplinary characterization.

Smelting slags is a well-known industrial solid waste, while there were limited studies on the key factors controlling the potential health risks caused by these smelting slags. In this work, the metal bioaccessibility in the size fractionated-zinc smelting slags was examined using various In vitro assays, in combination with multidisciplinary methods. The results indicated that the bioaccessible fractions of heavy metals showed a significant difference, but no statistical difference among different particle sizes of the zinc smelting slags. The bioaccessible metal fractions in the gastric (GP) and gastrointestinal (GIP) phases were 0 (Cr) - 91.39% (Cd)) and 0 (Cr) - 47.80% (Ni). Among the studied metals, Cd, Cu, Mn, Pb and Zn were the most bioaccessible to human. The Pearson correlation analysis showed that the carbonate bound phases of heavy metals were responsible for their bioaccessibility in GP and GIP. Moreover, the combined results of multidisciplinary characterization also further implied that the solubility behaviors of toxic elements in the smelting slags were dominated by soluble metal bearing- mineral phases and absorbable Fe, Mn and Al-rich minerals and metal bearing-precipitates during SBRC extractions. Therefore, these study results provide a insight into the potential controls of metal bioaccessibility in the zinc smelting slags, which was of great significance from the aspects of their resource recycling and risk management.

Bioavailability Assessment of Heavy Metals Using Various Multi-Element Extractants in an Indigenous Zinc Smelting Contaminated Site, Southwestern China.

Despite recent studies have investigated the strong influences of smelting activities on heavy metal contamination in the soil environment, little studies have been conducted on the current information about the potential environmental risks posed by toxic heavy metals in smelting contaminated sites. In the present study, a combination of the bioavailability, speciation, and release kinetics of toxic heavy metals in the indigenous zinc smelting contaminated soil were reliably used as an effective tool to support site risk assessment. The bioavailability results revealed that the bioavailable metal concentrations were intrinsically dependent on the types of chemical extractants. Interestingly, 0.02 mol/L EDTA + 0.5 mol/L CH3COONH4 was found to be the best extractant, which extracted 30.21% of Cu, 31.54% of Mn, 2.39% of Ni and 28.89% of Zn, respectively. The sequential extraction results suggested that Cd, Pb, and Zn were the most mobile elements, which would pose the potential risks to the environment. The correlation of metal bioavailability with their fractionation implied that the exchangeable metal fractions were easily extracted by CaCl2 and Mehlich 1, while the carbonate and organic bound metal fractions could be extracted by EDTA and DTPA with stronger chelating ability. Moreover, the kinetic modeling results suggested that the chemical desorption mechanism might be the major factor controlling heavy metal release. These results could provide some valuable references for the risk assessment and management of heavy metals in the smelting contaminated sites.

[Enhanced Electrokinetic Remediation of Heavy Metals Contaminated Soils by Stainless Steel Electrodes as well as the Phenomenon and Mechanism of Electrode Corrosion and Crystallization].

Electrode corrosion and salt crystallization are important challenges that restrict the engineering application of electrokinetic technology. In the present study, using stainless steel as an electrode, and deionized water (DW), citric acid (CA) and polyaspartic acid (PASP) as electrolytes, Pb/Cu-contaminated soil was remediated by electrokinetic. All of the EK experiments were conducted in a 2 L soil cell reactor with a moisture level of about 35% blended with 1000 mg·g-1 of Pb and 778 mg·g-1 of Cu under a constant voltage gradient (1 V·cm-1, 2 V·cm-1) for 150 h. The removal efficiency of heavy metals and influencing factors, as well as the phenomenon and mechanism of electrode corrosion and salt crystallization were explored. The experimental results showed under the action of electric field, the Ca in the test soil would move to the cathode, and the crystal was formed in the alkaline condition. Additionally, the conductivity of the electrode was reduced. During the EK process, water at the anode was primarily oxidized, undergoing a reduction reaction at the cathode. Because H+ and OH- were transported through the soil by electromigration and electro-osmotic flow (EOF), changes in soil pH could occur. The concentrations distribution of Pb-Cu appeared to be related to the distribution of soil pH in the cell, which might be associated with the desorption and hydroxide precipitation of Pb-Cu. PASP resulted in obvious inhibitory effect on the corrosion of stainless steel electrode, CA and PASP could clearly destroy the formation of CaCO3 crystal, while barely effectively disrupted the formation of Ca (OH)2 crystal. Both CA and PASP could promote the removal of Pb, but the influence of PASP on the removal of Cu was not obvious, and the effect of CA was very significant. Combined with different corrosion inhibitor and reinforcing agent, stainless steel can be chosen as the engineering application electrode in electrokinetic remediation.

[Optimization of electrode configuration in soil electrokinetic remediation].

Electric field distributions of several different electrode configurations in non-uniform electric field were simulated using MATLAB software, and the electrokinetic remediation device was constructed according to the best electrode configuration. The changes of soil pH and heavy metal residues in different parts of the device during the electrokinetic remediation were also studied. The results showed that, in terms of the effectiveness of the electric field strength, the square (1-D-1) and hexagonal (2-D-3) were the optimal electrode configurations for one-dimensional and two-dimensional respectively and the changes of soil pH, the removal of heavy metals and the distribution of electric field were closely related to one another. An acidic migration band, which could prevent premature precipitation of heavy metals to a certain extent and promote electrokinetic removal of heavy metals, was formed gradually along with the remediation in the whole hexagon device when the cathodic pH was controlled during the remediation of the four cationic metallic ions, Cd2+, Ni2+, Pb2+ and Cu2+. After 480-hour remediation, the total removals of Cd, Ni, Pb and Cu were 86.6%, 86.2%, 67.7% and 73.0%, respectively. Remediation duration and replacement frequency of the electrodes could be adjusted according to the repair target.

[Enhanced remediation of 4-chloronitrobenzene contaminated groundwater with nanoscale zero-valence iron (nZVI) catalyzed hydrogen peroxide (H2O2)].

Chemical oxidation-reduction technology is an important way to quickly remedy contaminated groundwater. Nanoscale zero-valent iron (nZVI) was produced by liquid-phase reduction using FeSO4 and NaBH4, and characterized by SEM and XRD. The remediation of 4-chloronitrobezene (4-CINB) contaminated groundwater at ambient temperature and pressure was conducted with the nZVI catalytic H2O2 process, and the affecting factors and degradation mechanisms were investigated. The results indicated that under initial pH 3.0 at the temperature of 30 degrees C, after 30 mins of reaction, 4-ClNB in groundwater was completely degraded when the concentrations of nZVI and H2O2 were 268.8 mg x L(-1) and 4.90 mmol x L(-1), respectively. 4-chloronitrosobenzene, 4-chlorophenylhydroxylamine, 4-chloroazoxybenzene, 4-chloroaniline, 4-chloroazobenzene, 4-benzoquinone, acetic acid, formic acid, oxalic acid and chlorine ion were identified as the major intermediates of 4-ClNB degradation after the process. A tentative pathway for the degradation of 4-ClNB was proposed.

Toxicological effect of joint cadmium selenium quantum dots and copper ion exposure on zebrafish.

Quantum dots (QDs) have strong adsorption capacity; therefore, their potential toxicity to aquatic organisms from the facilitated transport of other trace toxic pollutants when they coexist has received increasing interest. However, the impact of cadmium selenium (CdSe) QDs and copper ion (Cu(2+)) joint exposure on zebrafish (Danio rerio) embryo and larvae remains almost unknown. Therefore, the present study was performed to determine the developmental toxicities to zebrafish exposed to combined pollution with CdSe QDs (500 µg/L) and Cu(2+) (0, 0.1, 1, 10, and 100 µg/L CuC1(2)) compared with single exposure. Our findings for the first time revealed that: (1) QDs facilitated the accumulation of Cu(2+) in zebrafish; (2) QDs caused higher mortality, lower hatch rate, and more malformations of the exposed zebrafish; (3) junction, bifurcation, crossing, particles, and aggregation of the exposed FLI-1 transgenic zebrafish larvae can be observed; (4) embryo cell apoptosis appeared in the head and tail region; and (5) synergistic effects played an important role during joint exposure. These observations provide a basic understanding of CdSe QDs and Cu(2+) joint toxicity to aquatic organisms and suggest the need for additional research to identify the toxicological mechanism.

[Remediation of chromium (VI) contaminated soils using permeable reactive composite electrodes technology].

Electrokinetic transport processes have been shown to have potential for the effective removal of heavy metals from soils. However, pH changes near the anode and cathode limit their widespread application in the remediation of contaminated soils. Permeable reactive composite electrodes (PRCE) were made by attaching reactive materials such as Fe(0) and zeolite to the electrodes, and the effects of the composite electrodes on pH control, chromium removal efficiency and Cr speciation changes were studied in the electrokinetic remediation process of Cr( VI) contaminated soil. Composite electrodes consisting of permeable reactive materials gave better pH control and Cr removal efficiency compared to traditional electrodes, and a Fe(0) + zeolite reactive layer in the anode exhibited the best performance compared to zeolite or Fe(0) alone. After 5 days of electrokinetic remediation with a DC voltage of 2 V x cm(-1), the Fe(0) + zeolite reactive layer lowered the pH fluctuation, maintained the soil pH in the range of 5.5 to 8.5, raised the Cr(VI) removal efficiency up to 97% in any soil section, produced lower Cr(III) residues, enhanced the amount of Cr retention up to 8 and 1.8 times respectively, and transformed 98% of the Cr(VI) into lower toxicity Cr(III). This study provides a theoretical basis for the exploitation of permeable reactive composite electrodes which are a practical option for future applications.

[Stabilization of heavy metals in sewage sludge by using a ferrous iron].

The stabilization of heavy metals in sewage sludge using a ferrous iron is investigated through leaching test and fractionation analysis. The results showed that FeSO4 x 7H2O was not helpful to stabilize the heavy metals in the sludge when used individually since it reduced the pH value of sludge. The pH value dropped significantly from 6.5 to 4.4, and leaching coefficients of Zinc (Zn) and Nickel (Ni) raised, respectively, to 363.3% and 118.3% when FeSO4 x 7H2O was added individually at a ratio of 5%. However, it maintained the pH value within a slight alkaline range and the leaching coefficients of heavy metals reduced significantly when combined with Ca(OH)2 as an auxiliary agent. The pH value of sludge kept to 8.4 and leaching coefficients of Zn and Ni were reduced to 11.5% and 24.1%, for example, when 15% FeSO4 x 7H2O and 7.5% Ca(OH)2 were added together into the sludge. Besides, when Ca(OH)2 was used individually, the pH value of the sludge raised highly although the heavy metals were stabilized effectively. Under same pH value, combined addition of FeSO4 x 7H2O and Ca(OH)2 promoted the stabilization of heavy metals had a better effect than individual use of Ca(OH)2. The fractionation analysis revealed that the stable species of Zn, Cu and Pb were increased, and the instable species of Ni, Cr and Cd were also raised.

[Application of solidification technology in ecological protection of rural riverbank].

A self-developed binder was used for the solidification of construction refuse piles and whole soil matrix, and a technology of this solidification combining with grass-planting was adopted to ecologically protect the rural riverbanks at Tianshan Village of Shanghai. This technology and other ecological engineering techniques were also employed to reconstruct the ecological environment of a sewage pond at the Village. The results showed that the solidified piles had an anti-compression strength of up to 7.3 MPa, with good hydraulic permeability, fast hardening rate, and low drying shrinkage, which met the requirements for ecological safety. The solidified stakes could be used at a low temperature of above -18 degrees C with addition of certain anti-freezing agents. The riverbank underpinned with the solidified stakes had higher anti-compressive strength, higher ability of anti-soil erosion, and better hydraulic permeability; and its soil had the similar moisture content to bare riverbank soil, with no detrimental effects on the root growth of planted grass. After soil solidification, the shearing strength of the riverbank increased by 50 times, and its soil loss was only 5% of the bare riverbank. In the first 10 days after adopting this technology, parts of Cynodon dactylon roots on the surface of solidified soil matrix began to extend into soil; after one month, 60% of the roots penetrated into deeper soil layer; and 11 months later, the grass roots completely grew in-depth in the soil. The combination of our solidification technique with vegetation reconstruction satisfied the requirements of both stabilizing riverbank and improving riparian habitat.

[Characteristics of microbial biomass in subsurface constructed wetland treating eutrophic water].

A subsurface horizontal-flow constructed wetland planted with Phragmites australis was developed and used to treat eutrophic water for nearly two years at fixed hydraulic loading rate. Substrate samples were taken at different depths respectively in the front, middle and back sites of wetland in January, May, August and October in the second year. Microbial biomass (MB) content was measured using the chloroform fumigation incubation method. The results show that the front sites have higher levels of microbial biomass carbon (MB-C), microbial biomass nitrogen (MB-N) and microbial biomass phosphorous (MB-P) than that in middle sites and back sites. The upper layers have higher levels of MB than that in the deeper layers. The MB content of wetland in January and May is higher than that in October and August. The relationship between MB and TN, TP removal efficiency of the constructed wetland was investigated. The wetland shows no apparent correlation of MB and TN removal efficiency. However, strong negative correlation is observed between MB-C and TP removal efficiency (r = -0.98, p < 0.05) and between MB-N, MB-P and TP removal efficiency (r = -0.99, p < 0.01). In constructed wetland, MB is also an active nutrient storage involved in nutrient cycling and can be used by wetland plant. MB content varies among a range level with plant growth and temperature in a growth year. Under experiment conditions, the range of MB-C, MB-N and MB-P was respectively 85.0-160.6, 16.3-34.9 and 3.12-5.77 microg x g(-1) in the second year. Seasonal variation of MB and nutrient removal is resulted from the factors integrated with temperature, microorganisms, plant, substrate and enzyme.