Behavior Of Heavy Metals Research Articles

Different crop rotation patterns vary heavy metals behavior in soils under plastic sheds: Evidence from field research

Facility agriculture improves food supply worldwide. However, heavy metals accumulation in its soils under plastic sheds increases the contamination risk, and little is known about metal’s behavior in soils under different rotation patterns. This study investigated total and available Cd, Cu, Ni, Pb, and Zn in soils under plastic sheds from tomato-cropping (TCC), tomato-leaf vegetable rotation (TVC), tomato-melons rotation (TMC), and fruit-cropping (FLC). Results indicated that accumulations and bioavailability of Cd, Cu, and Zn were severe, and risks of tomato-based rotations were higher than FLC. Geo-accumulation index showed that accumulation of Cu and Zn in soils from TCC was the worst, and Cd in TVC was the highest. Principle-component analysis suggested Cu and Zn might be from manures. Linear regression analysis showed significant annual increases of total Cu and Zn in TCC; Cd in FLC and available Cd in TVC should be taken seriously. Different fertilizers and dry matter removal might be reasons for different behaviors of metals. The lowest amounts of both factors in FLC make its soils healthier. Tomato-based patterns should consider more intercropping and rational fertilization, and FLC might be an alternative. Proper fertilization and selection of rotation patterns can prompt soil health of facility farmlands.

Stabilization of heavy metals in municipal solid waste incineration fly ash via hydrothermal treatment with coal fly ash

The environmental risk of heavy metals in hazardous municipal solid waste incineration fly ash (FA) is one of the most important concerns for its safely treating and disposing. This study investigated the stabilization behavior of heavy metals in FA using coal fly ash (CFA) as an additive via hydrothermal treatment. The effects of water washing pre-treatment and FA/CFA ratio on leaching behavior, speciation evolution, and risk assessment of heavy metals were studied. The results showed that 96.6–98.0 % of Cl can be effectively removed by water washing pre-treatment and hydrothermal treatment. Most heavy metals (Cr, Cu, Ni, Pb and Zn) (>91.5 %) were stabilized in the hydrothermal product, rather than transferred to liquid phase. Tobermorite can be synthesized by adjusting Ca/Si ratio with the addition of CFA. The heavy metals were transferred into more stable residue fractions with increasing CFA addition, which resulted in the significant reduction of leaching concentrations and risk assessment code (RAC) of heavy metals. Among, the product with 30% CFA exhibited the most superior performance with the lowest leaching concentrations of heavy metals and RAC was at no risk level (<1). In addition, the economic performance of hydrothermal treatment exhibited a potential advantage by comparing with FA-to-cement, FA-to-glass slags and FA-to-chelating agent & cement solidification/stabilization. Therefore, the hydrothermal treatment coupled with water washing pre-treatment would be a promising method for the detoxification of FA, as well as synergistic treatment of FA and CFA.

Solar Drying as an Eco-Friendly Technology for Sewage Sludge Stabilization: Assessment of Micropollutant Behavior, Pathogen Removal, and Agronomic Value

Sewage sludge (SS) is a biosolid that includes nutrients, organic matter, and a mixture of micropollutants and pathogens. Regarding its final disposal, several criteria should be met to avoid the dissemination of the included micropollutants in the environment. Hence, an adequate treatment prior to SS disposal is highly required. Solar drying is being acknowledged as a sustainable process of SS treatment, yet it is still unclear to what extent this technique is efficient. This review aimed to assess the impact of solar drying on the composition of SS from environmental and agronomic standpoints. Herein, we present the state-of-the-art with regard to solar drying efficiency in terms of water content reduction, DM increase, agronomic parameters evolvement, and micropollutant stabilization including pathogens, heavy metals (HMs), and organic micropollutants. The reviewed literature is mostly focused on two drying cycles: summer and winter, thus addressing the extreme conditions met within a year with respect to temperature. Under different climatic conditions, more than 80% of dry matter is reached during summer. In winter, the efficiency decreases to an average of 50% of DM. Negatively correlated to DM content, pathogen concentration in SS significantly decreased, while DM increased. Thus, more efficiency in terms of pathogen abatement is reported in summer than in winter (e.g., 96% against 60% during summer and winter, respectively, under semi-arid climate). The high reliance of solar drying efficiency on weather has been deduced in terms of DM content increase and pathogen removal. Where climatic conditions are not favorable for solar drying, hybrid design and liming are the highly recommended methods to remove pathogens from SS. A few studies on the fate of HMs in SS during solar drying concluded that solar drying does not involve any removal mechanisms. Changes in HM speciation in solar-dried sludge were reported highlighting a decrease in their mobility. As for organic micropollutants (PAHs and antibiotics), only their occurrence in SS is reported in the literature, and their behavior during the solar drying process is still not addressed. This review allowed concluding the following: 1) solar drying is a sustainable, relevant process for SS handing in terms of volume reduction and pathogen removal, particularly in semi-arid regions; 2) solar drying does not lower the SS agronomic value and does not remove HMs, but under semi-arid climate, it changes HM speciation and reduces their mobility. The gap in research regarding organic micropollutant and heavy metal behavior during SS solar drying has been emphasized as a way forward for research within this topic. Hence, more research is required to help stakeholders decide on the feasibility of an agricultural disposal of solar-dried sludge.

Environmental Risk and Management of Herbal-Extraction Residues Induced by the Composition and Metal Binding Properties of DOM

As the consumption of Chinese medicine resources increases, the waste of traditional Chinese medicine extraction cannot be disposed of reasonably, which has a serious impact on the environment. Dissolved organic matter (DOM), a crucial fraction in herbal extraction residue, can bond to heavy metals (HMs), creating a potential environmental risk. This study investigated the binding property of herbal extraction residue DOM with Cu(II) via two-dimensional Fourier transform infrared (FTIR) and synchronous fluorescence correlation spectroscopy (2D-COS) in conjunction with parallel factor (PARAFAC) modeling. Through PARAFAC analysis, three kinds of protein components and one kind of fulvic acid can be obtained, and protein-like substances are dominant in the residual DOM of Chinese medicine extracts, becoming the main factor of water quality deterioration. A fluorescence quenching experiment shows that protein-like materials provide the primary binding sites with Cu(II). During the detection, the long-wavelength low-intensity signal will be obscured. The 2D-COS obtained by the 1/9th power transformation can enhance the fluorescence signal so as to get even more information about the binding sites and heterogeneity of DOM and heavy metal ion ligands. The N−H of amine in the protein-like materials could prioritize combining with Cu(II). This study urges that continuous and effective attention be paid to the impact of the herbal extraction residue on the geochemical behavior of HMs.

Spatial and temporal variations of metal fractions in paddy soil flooding with acid mine drainage

Environmental release of acid mine drainage (AMD) poses a potential threat to the environment and human health due to its high content of heavy metals. The impact of AMD flooding on unpolluted soil leads to serious pollution over time via a complex process, related to the geochemical behavior of toxic metals that so far has only been partially investigated. Here, a soil column study was conducted to investigate the migration of Cu and Cd fractions in unpolluted paddy soil following treatment with AMD collected from the Dabaoshan Mining area. Tessier's sequential extraction was performed to fractionate the metals at various depths over time. After 160 days of experimental flooding, the soil pH stabilized at 2.52 at a column depth of 5 cm. The fractions of Cu and Cd that were highly mobile increased significantly during AMD flooding. For Cd, the latter already occurred on day 67. At a depth of 20 cm, the total content of Cu maximally increased from initially 26.89 mg kg−1 to 696.96 mg kg−1 on day 160, while the content of Cd maximally increased from 0.22 mg kg−1 to 391.30 mg kg−1 on day 67. Reduced partition index analysis conformed that the mobility of both Cu and Cd significantly increased in contaminated soil during continuous AMD flooding. Scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS) identified a changed distribution of the elements in the soil, with Fe appearing to have aggregated. The correlation analysis between Cu and Cd in pore water and in different fractions in the soil's solid phase identified a dynamic distribution of these metals in certain geochemical components during their migration. The results of this study contribute to a scientific foundation to describe the geochemical behavior of heavy metals in soil subject to AMD flooding.

Drying and rewetting induce changes in biofilm characteristics and the subsequent release of metal ions

Drying and rewetting can markedly influence the microbial structure and function of river biofilm communities and potentially result in the release of metal ions from biofilms containing metals. However, little information is available on the response of metal-enriched biofilms to drying and rewetting over time. In this study, natural biofilms were allowed to develop in four rotating annular bioreactors for 2–11 weeks, followed by drying for 5 days and rewetting for another 5 days. Subsequently, we assessed Zn, Cd, and As desorption from the biofilms and other related parameters (microbial community structure, biofilm morphology, enzyme activity, and surface components as well as characteristics). High-throughput sequencing of the 16 S rRNA gene and confocal laser scanning microscopy revealed that the biofilm architecture and bacterial communities were distinct in different growth phases and under drying and rewetting conditions (permutational multivariate analysis of variance; p = 0.001). Proteobacteria was the dominant bacterial phylum, accounting for 69.7–90.1% of the total content. Kinetic experiments revealed that the drying and rewetting process increased metal desorption from the biofilm matrix. The desorption of heavy metals was affected by the age of the biofilm, with the maximum amount of metal ions released from 2-week-old biofilms (one-way ANOVA, Zn: p < 0.001; Cd: p = 0.008; As: p < 0.001). The modifications in biofilm properties and decreased diversity of the bacterial community (paired t-test, p < 0.05) after drying and rewetting decreased the number of specific binding sites for metal ions. In addition, negatively charged arsenate and other anions in the liquid phase could compete with As ions for adsorption sites to promote the release of As(V) and/or reductive desorption of As(III). The results of this study and their interpretation are expected to help refine the behaviors of heavy metals in the aquatic environment.

New insight into the role of FDOM in heavy metal leaching behavior from MSWI bottom ash during accelerated weathering using fluorescence EEM-PARAFAC

Fluorescence excitation-emission matrix (EEM) spectroscopy is a powerful tool to characterize DOM that interacts with heavy metals in MSWI bottom ash (IBA). Here, two fresh IBA samples collected from large MSWI plants were subjected to 33 days of accelerated weathering. Carbon content and fluorescence characterization of DOM and leaching behavior of heavy metals (Cu, Ba, Cr, Ni, and oxyanions) were monitored during the weathering. The mineralogical and chemical properties of IBA during the weathering process were also characterized. EEM combined with parallel factor analysis showed that fluorescent DOM could be decomposed into humic-like (C1, C2) and tryptophan-like substances (C3), while the accelerated weathering process can be further divided into three phases. Fitted cubic polynomials described well the changes in the specific intensity of fluorescence components. Humification and freshness indexes and SUVA results suggested the leached DOM contained a higher proportion of condensed aromatic structures and/or conjugation of aliphatic chains post-weathering. The results also revealed that adsorption of humic-like substances onto neo-formed reactive surfaces occurred quickly in the early stage of accelerated weathering; thereafter, biodegradation of lower molecular mass-hydrophilic organic carbon fraction plays a vital role in further reduction of Cu and Cr leaching in subsequent weathering. Oxyanions (Mo and Sb) became more mobile after 3 days of accelerated weathering, but their leaching was effectively reduced after the weathering process. A novel method for an IBA weathering treatment combined with enhanced microbial degradation is proposed. These findings provide new and inspiration for improving accelerated weathering technology.

Dissolved oxygen drives the environmental behavior of heavy metals in coastal sediments.

In this study, the impacts of dissolved oxygen (DO) on dynamics concentrations of heavy metals (Cu, Cd, Cr, and Pb) from estuary sediments were investigated in a 49-day laboratory simulation. The exchange flux method, Bureau Communautaire de Référence (BCR) sequential extraction procedure, and risk assessment code (RAC) were used to analyze the behavior of heavy metals. The results indicated that oxic environments promoted the concentrations of Cu and Cd in overlying water compared to the anoxic environments. The exchange fluxes showed that the diffusion of Cu, Cd, Cr, and Pb from sediments was the predominant process in the first 9days, and a metastable equilibrium state was gradually reached in the later period under anoxic conditions. However, oxic conditions extended the time required to reach metastable equilibrium for Cu over the sediment-water (overlying water) interface (SWI). Although the reducible fractions of Cu, Cd, and Pb accounted for a large proportion of their total levels, the release ability of Cu, Cd, and Pb was limited by the high content of sulfide under anoxic conditions. The RAC values indicated that anoxic environments increased the proportion of acid-soluble fraction. The information obtained from this study highlights the potential risk for re-release of heavy metal from sediments under different redox conditions.

Multifaceted evaluation of distribution, occurrence, and leaching features of typical heavy metals in different-sized coal gasification fine slag from Ningdong region, China: A case study

The coal gasification fine slag (CGFS) from the entrained-flow coal gasification unit faces the challenge of safe disposal and clean utilization in the Ningdong region, China. This study aims to provide complete and thorough understanding of the distribution features, chemical speciation, environmental impact, and leaching behavior of typical heavy metals (i.e., V, Cr, Mn, Ni, Cu, Zn, Ba, and Pb) in the CGFS with different size fractions. The results show that the distribution of selected heavy metals in the CGFS has evident particle size dependence. Except for Zn, the other heavy metals in different size fractions mainly exist in chemical speciation of residual form with the ratio of 50.11–86.69 wt%. Moreover, it is found that the heavy metals in the different-sized CGFS show different RAC (risk assessment code) environmental risk levels and TCLP (Toxicity Characteristic Leaching Procedure) leaching concentrations. Especially, Zn in SGFS-C and SGFS-D posed a high-risk level to the environment, while the heavy metal elements of Cr, Mn, Ni, Zn, and Ba in other size fractions are classified as a medium environmental risk. In addition, the TCLP test results indicate that the leaching concentration of Cr, Mn, Ni, Zn, Ba, and Pb exceeds the groundwater-related regulatory limit in China. The pH-dependent leaching experiments suggest that Pb shows the amphoteric behavior, while the leaching mode of other heavy metals seems to be the cationic pattern. Furthermore, the leachability of the selected heavy metals in small-size fractions of the CGFS should be given more consideration at both acid and alkaline pH ranges. The leaching kinetic results demonstrate that the most effective mechanism to describe the leaching process of Cr, Ni, Zn, and Pb in different CGFS size fractions is the diffusion-controlled theory, which is supported by the different morphological traits of spherical mineral particles and carbon particles in the CGFS.

Fate and emission behavior of heavy metals during hazardous chemical waste incineration

The fate and emission behavior of heavy metals (As, Cd, Co, Cr, Cu, Ni, Pb, Se, and Zn) from a hazardous chemical waste incinerator were systematically explored. The results show that the main components of incineration fly ashes and slags contain minerals such as salt, plagioclase, pyroxene, gypsum, calcite, and slaked lime. The elements As, Cd, Pb, and Se are enriched in the fly ash particles during flue gas condensation. Co and Ni are more likely to be deposited in the rotary kiln slag and cooling tower slag owing to their lower volatility. Zn, Cr, and Cu are usually volatilized into the flue gas as oxides or chlorides are condensed and enriched in the slag of the cooling tower during the flue gas cooling process. The content of As, Cd, Pb, Ni, Cr, and Se increase with decreasing fly ash particle size. After the flue gas purification equipment was employed, the concentration of particulate metals significantly reduced. In the exhaust flue gas, the concentrations of Cu and Zn are 29.85 and 28.47 μg/m3, those of As, Cr, Ni, Pb, and Se range from 2.54 to 9.25 μg/m3, and those of Co and Cd are 0.42 and 0.13 μg/m3, respectively.

Co-Pyrolysis of Sewage Sludge and Wetland Biomass Waste for Biochar Production: Behaviors of Phosphorus and Heavy Metals.

Large amounts of sewage sludge (SS) and wetland plant wastes are generated in the wastewater treatment system worldwide. The conversion of these solid wastes into biochar through co-pyrolysis could be a promising resource utilization scheme. In this study, biochar was prepared by co-pyrolysis of SS and reed (Phragmites australis, RD) using a modified muffle furnace device under different temperatures (300, 500, and 700 °C) and with different mixing ratios (25, 50, and 75 wt.% RD). The physicochemical properties of biochar and the transformation behaviors of phosphorus (P) and heavy metals during the co-pyrolysis process were studied. Compared with single SS pyrolysis, the biochar derived from SS-RD co-pyrolysis had lower yield and ash content, higher pH, C content, and aromatic structure. The addition of RD could reduce the total P content of biochar and promote the transformation from non-apatite inorganic phosphorus (NAIP) to apatite phosphorus (AP). In addition, co-pyrolysis also reduced the content and toxicity of heavy metals in biochar. Therefore, co-pyrolysis could be a promising strategy to achieve the simultaneous treatment of SS and RD, as well as the production of value-added biochar.

Migration behavior of heavy metals in coal fly ash-derived Cu-SSZ-13 during synthesis and NH3-SCR application

The hazardous heavy metals in coal fly ash (CFA) restrict the recycling, utilization, and application of CFA and its derived products, especially for the CFA-fabricated zeolite that is applied to environmental catalysis. To clarify the migration behavior of the heavy metals during the synthesis process of target zeolite and its application, we prepared zeolite SSZ-13 using a two-step hydrothermal strategy and evaluated the migration and residual rates of the heavy metals including Hg, As, Se, Pb, Cr, and Cd in these processes. The NH3-SCR of NO performance and its effects on the existence of heavy metals were also examined. A modified Community Bureau of Reference (BCR) sequential extraction procedure was used to extract the heavy metals. Pb, Cr, and Cd remain in the SSZ-13, while Hg, As, and Se reside in the leachate during the hydrothermal reaction. The former three elements existed mainly as residuals in zeolite SSZ-13. The latter moved to the leachates. The SSZ-13 after ion-exchanged with Cu2+ shows high NH3-SCR performance, and the residual minim heavy metals rarely moved into the environment, indicating a harmless utilization process. This work contributed to the harmlessness and recycling of CFA and the development of high-value CFA-derived zeolite catalysts applied to environmental catalysis.

Roles of Heavy Metals during Pyrolysis and Gasification of Metal-Contaminated Waste Biomass: A Review

The presence of contaminates (i.e., heavy metals) in biomass through various sources (industrial activities and municipal solid waste) from its growth to end use has become a nuisance. Over the past years, biofuel production from metal-contaminated waste biomasses (MCWBs) has attracted more attention on account of the potential roles in addressing both energy and environmental concerns. This review considers the migration and transfer behaviors of heavy metals into various output streams, including solid products (i.e., biochar), tar and syngas from MCWBs, i.e., contaminated energy crops, plants, industrial wastes, and municipal solid waste, and using thermochemical processes (gasification and pyrolysis). In addition, the authors also shed light on the transfer mechanisms of heavy metals (however, the availability of the literature on the transfer mechanisms of heavy metals during thermal process is very limited) and usability of products (bio-oil, biochar, and syngas), including a comparison of gasification and pyrolysis processes in regard to the conversion of MCWBs into bioenergy. In the end, techno-economic assessments and future perspectives are presented. This review will be helpful to promote future research and may aid in the scale-up of the current practices to visualize MCWBs as potential opportunities instead of nuisances.

Effect of sintering temperature on solidification and migration of non-volatile and volatile heavy metals in glass-ceramic

Using tailings as material to prepare glass-ceramic is an excellent way to achieve the resource utilization of solid waste. However, at present, researches on the solidification and migration of heavy metals are limited. Therefore, in this study, ten groups of samples were prepared by controlling sintering temperatures. The solidification, migration, and leaching behavior of non-volatile and volatile heavy metals were studied. The research showed that, with the increase of temperature, the properties of the samples were improved. Fe participated in the phase transformation and evolved into insoluble iron pyroxenes solid solution, while Pb was homogeneously distributed in the glass matrix of glass-ceramics. The leaching concentrations of Fe and Pb in the glass-ceramics were 0.055 mg/L ∼0.087 mg/L and 0.074 mg/L ∼0.140 mg/L, which were far below the threshold value. The results showed that heavy metals can be effectively solidified in glass-ceramics and have good environmental benefits.

Multi-spectroscopic investigation of the molecular weight distribution and copper binding ability of dissolved organic matter in Dongping Lake, China

The properties and metal-binding abilities of dissolved organic matter (DOM) rely on its molecular weight (MW) structure. In this study, the spatial differences of DOM in compositions, MW structures, and binding mechanisms with copper (Cu2+) in Dongping Lake were investigated by applying excitation-emission matrix combining parallel factor analysis (EEM-PARAFAC), synchronous fluorescence (SF) spectra, two-dimensional correlation spectra (2D-COS), and Fourier transform infrared (FTIR) spectra. The EDOM for the entrance of the Dawen River and PDOM for the macrophyte-dominated region were divided from DOM of Dongping Lake based on hierarchical clustering analysis (HCA) and principal component analysis (PCA) and were size-fractioned into MW < 500 kDa and <100 kDa fractions. According to EEM-PARAFAC, Dongping Lake was dominated by tryptophan-like substances with MW < 500 kDa. The concentration of PDOM was higher than that of EDOM (p < 0.05). 2D-COS showed that protein-like components preceded humic-like components binding to Cu2+ regardless of sample type (215 nm > 285 nm > 310–360 nm). The Cu2+ binding capacity of DOM exhibited specific differences in space, components, and molecular weights. The humic-like component 1 (C1) and tryptophan-like component 4 (C4) of PDOM showed stronger binding abilities than those of EDOM. Endogenous tryptophan-like component 4 (C4) had a higher binding affinity for Cu2+ than humic-like components (logKa: C4 > C1 > C2) in PDOM irrespective of MW. Humic-like components with MW < 500 kDa displayed higher binding potentials for Cu2+. FTIR spectra showed that the main participants of DOM-Cu complexation included aromatic hydrocarbons, aliphatic groups, amide Ⅰ bands, and carboxyl functional groups. This study provides spatial-scale insights into the molecular weight structure of DOM in influencing the behavior, fate, and bioavailability of heavy metals in lakes.

Spatial distribution and ecological risk assessment of heavy metals in karst soils from the Yinjiang County, Southwest China.

BackgroundSoil heavy metals (HMs) under different land-use types have diverse effects, which may trigger the ecological risk. To explore the potential sources of HMs in karst soils, the spatial distribution and geochemical behavior of HMs based on different land-use types are employed in this study.MethodsSoil samples (n = 47) were collected in three suites of karst soil profiles from the secondary forest, abandoned cropland and shrubland in Yinjiang, Southwest China. The concentrations of Ni, Mn, Cr, Pb, Cd and Mo were determined to give a comprehensive understanding of the possible sources of these HMs and evaluate the potential ecological risk in Yinjiang County.ResultsThe mean concentrations of HMs in all profiles followed the same order: Mn > Cr > Ni > Pb > Mo > Cd. Meanwhile, the concentrations of most HMs roughly increased with the depth. Additionally, the concentrations of HMs were mostly correlated with soil pH and SOC, rather than with clay and silt proportions. By contrast, with the enrichment factors (EF), geo-accumulation (Igeo) and potential ecological risk index (PERI) of HMs in soil under different land-use types, the results indicated that these HMs exhibited non-pollution (Igeo < 0) and no ecological risk (PERI < 30) to human health in soils of Yinjiang County.ConclusionsThe distribution of HMs is dominated by weathering in the karst area, and the effects of agricultural inputs on the enrichment of soil HMs in Yinjiang County are limited. This further state that the arrangement of the local agricultural structure is reasonable.

A review on heavy metal ions adsorption from water by layered double hydroxide and its composites

• We selected to review topics related to the specific application of LDH removal of heavy metals. • Classify and summarize its modification methods, absorption properties, composition, structure, and other aspects. • It is determined that future research needs to reduce environmental impact and economic costs to promote the synthesis and use of LDH. • This system summary will provide helpful guidance for preparing high-efficiency LDHs-based adsorbents and offer critical clues to elucidating the adsorption behavior of heavy metals on LDHs-based materials. Heavy metal pollution is highly toxic and persistent, posing a severe threat to human health, agriculture, and the safe environment. Therefore, there is an urgent need for fast and efficient heavy metal removal technology to deal with emergency response to heavy metal leakage. In recent years, the development of a unique layered double hydroxide material has attracted widespread attention in sewage treatment. Due to these fundamental scientific issues, the general concern about environmental pollution management of LDH-based materials, and the recent significant developments in this field, it is necessary to conduct a thematic review. This article summarizes 474 related articles published since 2005 and outlines functionalized layered double hydroxide (LDH) research status as heavy metal adsorption materials. In addition, there is still a lack of reviews on the adsorption characteristics, interaction mechanism, and application of LDHs-based nanomaterials in heavy metal removal. Here, the primary purpose of this article is to systematically summarize LDHs-based materials and their applications in removing heavy metals from aqueous solutions ( Fig. 1 ). First, we will focus on its preparation and modification methods to understand LDH intuitively. Subsequently, the effects of different environmental conditions on factors such as pH, temperature, and contact time were summarized. Finally, the interaction mechanism between LDHs composites and heavy metals is briefly described, and a quick conclusion is given.

Phase distribution, migration and relationship of polychlorinated dibenzo-p-dioxins and dibenzofurans and heavy metals in a large-scale hazardous waste incinerator

Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) and heavy metals emitted from hazardous waste incinerators have aroused public concern due to the risks to the environment and human health. In the study, phase distribution, migration, and relationship of PCDD/F and heavy metals (arsenic, cadmium, chromium, copper, mercury, nickel, plumbum, zinc) are investigated in a typical hazardous waste incinerator. The PCDD/F mass concentrations and toxic equivalent quantity are reduced to 3.26 ng/Nm3 and 0.0484 ng/Nm3 by air control pollution devices, with 93.1% remove efficiency. High sulfur content in hazardous waste results in low PCDD/F and heavy metals emissions. Moreover, migration analysis of PCDD/F mass concentrations indicates that vast PCDD/F migrates to the mixtures with sodium bicarbonate and activated carbon (52.4%), followed by fly ash (21.8%), sodium hydroxide solution (19.3%), and atmosphere (6.49%). Furthermore, TCDF isomers (37%) dominate among the PCDD/F congeners, while 1,2,3,4,6,7,8-HpCDF (20%) dominates among 2,3,7,8-substituted congeners. Deduced by the ratio of PCDF/PCDD, the dominant mechanism of PCDD/F is de novo synthesis and chlorobenzene route synthesis. Based on the relative enrichment index, heavy metals are classified into three types: volatile heavy metals (plumbum, zinc, cadmium, mercury), medium volatile heavy metals (copper, chromium, arsenic), involatile heavy metals (nickel). Additionally, copper is inclined to deposit into the bottom slag as CuSO4 rather than the volatile substance due to high levels of sulfur dioxide (3000 parts per million). The migration behavior of heavy metals is dependent on the volatility properties of these metals. Statistically, PCDD/F emissions have the highest correlation coefficient with plumbum (r = 0.815), followed by nickel (r = −0.798). Besides, the PCDD/F formation is promoted by the volatile heavy metal, prevented by medium volatile heavy metals. The study not only provides a comprehensive understanding of PCDD/F and heavy metals characteristics but also effective and feasible strategies to reduce the toxic pollutants emissions in large-scale hazardous waste incinerators.

Humic acids restrict the transformation and the stabilization of Cd by iron (hydr)oxides

Iron (hydr)oxides and their association with organic matter significantly affect the mobility of heavy metals in natural soils and sediments. However, the behavior of cadmium (Cd) during crystalline iron (hydr)oxide formation in the presence of humic acid (HA) is still unknown. In this study, the speciation of Cd in iron (hydr)oxide-HA coprecipitates were studied by extraction, surface complexation model (SCM) calculation and characterization of the composites during the aging. The results showed that aging promoted the stabilization of ~30–50% of the added Cd ions with minerals in the binary iron (hydr)oxide systems. The reduction of Cd occurred earlier than hematite formation, indicating that the aggregation of amorphous iron (hydr)oxide led to the initial immobilization of Cd. The presence of HA restricted the crystallization of iron (hydr)oxide by the formation of tight mineral nanoparticle-HA aggregates, while there were negligible changes in the speciation of Cd and Fe during aging at high HA concentrations. Therefore, HA promoted the adsorption of Cd onto amorphous iron (hydr)oxide but limited the partition of Cd to mineral aggregates. The knowledge about the role of HA in iron (hydr)oxide transformation and Cd speciation is of great significance for the prediction of heavy metal behavior in soils and sediments.

Accumulation and ecotoxicological risk assessment of heavy metals in surface sediments of the Olt River, Romania

Heavy metal pollution of river freshwater environments currently raises significant concerns due to the toxic effects and the fact that heavy metal behavior is not fully understood. This study assessed the contamination level of eight heavy metals and trace elements (Cr, Ni, Cu, Zn, As, Pb, Cd, and Hg) in the surface sediments of 19 sites in 2018 during four periods (March, May, June, and October) in Olt River sediments. Multivariate statistical techniques were used, namely, one-way ANOVA, person product-moment correlation analysis, principal component analysis, hierarchical cluster analysis, and sediment quality indicators such as the contamination factor and pollution load index. The results demonstrated higher contents of Ni, Cu, Zn, As, Pb, Cd, and Hg, with values that were over 2.46, 4.40, 1.15, 8.28, 1.10, 1.53, and 3.71 times more, respectively, compared with the national quality standards for sediments. We observed a positive significant statistical correlation (p < 0.001) in March between elevation and Pb, Ni, Cu, Cr, and Zn and a negative correlation between Pb and elevation (p = 0.08). Intermetal associations were observed only in March, indicating a relationship with river discharge from spring. The PCA sustained mainly anthropogenic sources of heavy metals, which were also identified through correlation and cluster analyses. We noted significant differences between the Cr and Pb population means and variances (p < 0.001) for the data measured in March, May, June, and October. The contamination factor indicated that the pollution level of heavy metals was high and significant for As at 15 of the 19 sites. The pollution load index showed that over 89% of the sites were polluted by metals to various degrees during the four periods investigated. Our results improve the knowledge of anthropogenic versus natural origins of heavy metals in river surface sediments, which is extremely important in assessing environmental and human health risks and beneficial for decision-maker outcomes for national freshwater management plans.