Migration Of Heavy Metals Research Articles

Effects of Combined Application of Biogas Slurry and Straw on the Migration and Fractions of Soil Heavy Metals in Rice-wheat Rotation System in Coastal Reclamation Areas

In order to reveal the impact of the application of biogas slurry instead of chemical nitrogen fertilizer on the environmental risk of heavy metals in the soil by returning straw to the field, four treatments, without biogas slurry and without straw applications (CK), biogas slurry application without straw (B), straw application without biogas slurry (S), and biogas slurry combined with straw applications (BS), were applied in a typical coastal reclaimed farmland (rice-wheat rotation) in Jiangsu province. The migration and morphological characteristics of Cu, Zn, Cd, and Pb in different soil layers were observed, and the potential environmental risks were estimated. The results showed that:① The total amounts of Zn and Pb in the surface soils (0-20 cm) in the rice and wheat fields under the BS treatment decreased significantly (P<0.05). The four heavy metals in the paddy soils migrated 6%-11% from the surface to the middle and lower layers (20-60 cm), and Cu, Cd, and Pb in the wheat soils migrate down from the surface by 25% to 33%. This indicated that the combined use of biogas slurry and straw accelerates the vertical downward movement of heavy metals in the surface soil. ② Under the BS treatment, the contents of the weak acid extraction of Cu in the surface soil of the paddy field decreased by 8.8%, and the residual state of Zn, Cd, and Pb decreased by 7.0% to 14.2%. This revealed that Cu was passivated, but Zn, Cd, and Pb tended to be activated. In comparison, the reduction in Cu residues in wheat field surface soil was 2.8 times that of the weak acid extraction, indicating that Cu was activated. Furthermore, the residue state of Cd increased, the weak acid extraction state of Pb decreased, and Cd and Pb were passivated. ③ The ecological risk assessment of heavy metals showed that there is no ecological risk in the soils under the BS treatment, and the risk indices were significantly lower than those of the B and S treatments (P<0.05). Therefore, the combined use of biogas slurry and straw helps to significantly reduce the risk of heavy metal pollution in the soils in the coastal reclamation areas.

Co-adsorption and interaction mechanism of cadmium and sulfamethazine onto activated carbon surface

Owing to their diverse functional groups, antibiotics can easily form complexes with heavy metals; the complexation alters the migration and transformation behavior of both antibiotics and heavy metals. In this study, we investigated the co-adsorption mechanism of sulfamethazine (SMT) and cadmium (Cd2+) heavy metal ions and created an ideal water model containing two major contaminants: sulfamethazine and heavy metal cadmium ions. Combined with the experimental analysis of the interaction mechanism, the results indicate the heterogeneous multilayer adsorption of SMT on the surface of activated carbon (AC). The bridging role of cadmium ions promotes the adsorption of SMT through the formation of SMT-Cd2+-AC ternary complexes on the surface of activated carbon. Characterization experiments provided further insight into the adsorption behavior of Cd2+ and SMT on activated carbon, revealing a strong correlation between the Cd2+-SMT complexation and the SMT adsorption capacity. These results indicate that the effects of the coexistence of antibiotics and heavy metal ions should be fully taken into account when investigating the environmental behavior of antibiotics and heavy metal ions.

Effect of Clay and Zeolite on Cd Migration in Soil-Plant Systems on Various Types of Natural Meadows

Nowadays, the special focus is placed on heavy metals. It is explained by the scope of their release into the environment and high toxicity. Moreover, heavy metals are able to enter into biological migration chains and, while entering into a body, affect it causing various types of mutations. Therefore, the search for measures to limit migration of heavy metals in various parts of the biological chain leading to the human has a great significance. The purpose of this paper was to study the effect of local clay and zeolite in the Khotynetskoe quarry of the Orel Region on Cd migration in the soil-plant system on natural meadows. To assess migrations, absorption indices were used that were calculated as the relation of average element contents values in plants (hay) to average values in the soil. During the studies it was found that not only applied ameliorates (clay, zeolite) but also their number affects Cd migration in the soil-plant system (hay).

Plastic shed production systems: The migration of heavy metals from soil to vegetables and human health risk assessment

Plastic shed production system (PSPS) provide abundant vegetable products for human consumption. Comprehensive and accurate heavy metal (HM) risk assessment of soil and vegetable under plastic sheds is crucial for human health. Pollution assessment, bioavailability and mobility evaluation and health risk assessment of Cd, Cr, Cu, Zn Ni, Pb, and As were performed in a presentative Plastic shed production system. The concentrations of the Cd, Cu and Zn exceeded their background value. Positive Igeo values suggested that soil under plastic sheds was widely contaminated with Cd. The bioavailability of heavy metals in soils was evaluated using DTPA extraction and DGT methods. The results of both methods demonstrated that Cd, Cu, and Zn have high bioavailability, especially Cd. Analogically, the results of mobility assignment based on DIFS showed that Cd has a high migration risk due to the large available pool. Based on specific cultivation and management patterns of plastic shed production system, pH reduction and salt and nutrient accumulation may increase the heavy metals migration risk in soil under plastic sheds, while a high organic matter content may reduce the heavy metals migration risk. The average concentrations of Cd, Cr, Cu, Zn, Ni, Pb, and As in vegetables were 0.023, 0.226, 0.654, 2.984, 0.329, 0.041, and 0.010 mg/kg, respectively. All samples were well below the threshold. The order of target hazard quotient of different heavy metals caused by vegetable consumption was Cd > Cr > As > Cu, Ni, Pb, Zn, and the average total hazard index value was below 1, which demonstrated that risk of vegetable consumption in the study area. However, due to its high concentration and transfer coefficient in spinach, Cd might pose a health risk to humans, which requires special attention. In this study, Cd caused a significant issue than other HMs, whether pollution level, health risk and migration risk. DGT and DIFS can be used as an effective evaluation tool in the research of controlling heavy metals migration in soil-crop systems.

Can incineration completely eliminate plastic wastes? An investigation of microplastics and heavy metals in the bottom ash and fly ash from an incineration plant.

The content of (micro)plastics and heavy metals were investigated in the fly ash, bottom ash and surface soil samples from a municipal solid waste incinerate plant. The abundance of microplastics was 23, 171, and 86 particles/kg dw, respectively. The type of microplastics in fly ash was fiber, and the main type in bottom ash and soil samples was fragment (43.0% and 29.3%), followed by film (26.3% and 25.0%), foam (13.0% and 25.1%), and fiber (17.7% and 20.7%). Most of the microplastics had obvious tearing marks, with the protrusions and scratches on their surfaces. Several types of heavy metals such as Cr, Cu, Zn, Pb were adsorbed on the surface of microplastics. Additionally, the column test demonstrated that the microplastics and heavy metals in the bottom ash can be significantly dissolved out under the impact of external precipitation. Results also indicated that acid rain precipitation easily dissolved heavy metals into the water environment from the bottom ash without special treatment or protection. This paper investigated the combined migration of microplastics and heavy metals from the bottom ash, which can provide theoretical basis for further study of properly treating the bottom ash and exploring the environmental behavior.

Experimental study on the stabilization of heavy metals in fly ash from municipal solid waste incineration by N-30 alkaline silica sol

The disposal of municipal solid waste incineration fly ash is a key environmental issue in China. The high content of heavy metals in fly ash makes ash disposal extremely difficult. A new method which binds N-30 alkaline silica sol and temperature was proposed for the first time in this study. The crystal composition and the patterns of heavy metal leaching concentration and speciation in fly ash were obtained via experiments. The results showed that adsorption of the self-made curing agent and the formation of metal silicate and phosphate promotes the migration of heavy metals from an unstable acid state to a stable residue state, thus improves the stability of heavy metals. The optimum temperature of self-made N-30 alkaline silica sol during treating process is about 700℃. Moreover, the risk assessment of heavy metal emission has proved that, the leaching amount of heavy metals after treatment reaches a risk-free level.

Effects of hydrophyte decomposition on the binding mechanism between fluorescent DOM and copper

Hydrophyte decomposition caused large amounts of dissolved organic matter (DOM) to enter aquatic environment that influence the migration and transformation of heavy metals (HMs). Six hydrophytes with five dry weight gradients (DWG) were used for the decomposition experiments. The results showed that protein-like materials occupy relatively high content in the hydrophyte-derived DOM. The binding properties of DOM-Cu(II) have been explored by using two-dimensional correlation spectroscopy (2D-COS) in conjunction with synchronous fluorescence spectroscopy (SFS) and log-transformed SFS. The weak signals of binding site can be amplified by the log-transformed 2D-COS analysis. Herein, more binding sites can be identified by the log-transformed 2D-COS analysis. The results reveal that tryptophan-like materials show a preferential sequence of binding Cu(II) in the hydrophyte-derived DOM with a relatively low DWG and sediment DOM, and fulvic-like substances indicate a preferential sequence of binding Cu(II) in the hydrophyte-derived DOM with a relatively high DWG. Meanwhile, the results of binding parameters indicate that the log K is the range of 3.61–4.25, 4.33–4.74, 4.59–4.97, 3.91–4.41, and 4.14–4.78 for D1-D5, respectively, suggesting that hydrophyte decomposition can change the binding affinity between DOM components and Cu(II). The complexes of fluorescent components with Cu(II) showed a high log K value at long wavelength (e.g. humic-like substances), and a relatively low fluorescent ligand proportion (f%) at shorter wavelength in the hydrophyte-derived DOM. However, the log K is the range of 3.08–4.31, 4.09–4.45, 3.93–4.35, 4.39–4.75, and 3.95–4.36 for C1-C5, separately. Protein-like substances with Cu(II) showed a relatively high log K value with the exception of C4. The log-transformed 2D-COS can be an analytical tool to understand the binding heterogeneity of DOM with HMs. The study can provide a guide for managing and controlling the effects of hydrophyte decomposition.

Enrichment and Migration of Heavy Metals in Mangrove Soil-Plant System from Sea Promenade in Zhanjiang

Enrichment and Migration of Heavy Metals in Mangrove Soil-Plant System from Sea Promenade in Zhanjiang

ASSESSMENT OF MIGRATION OF HEAVY METALS (Cd, Pb, As, Hg) IN THE SYSTEM «FEED - MILK»

The article presents the results on the level of migration of heavy metals in the food chain «feed-milk» of the farm «Aidarbayev», located in the Almaty region. A total of 56 feed samples and 30 milk samples were analyzed during the study period. Sampling from the facilities was carried out in the spring, summer and autumn of 2020 in accordance with the State Standards of the Republic of Kazakhstan and interstate standards. Work on determining the concentration of heavy metals was carried out on a novaa 350 atomic adsorption spectrometer with a talab voltammetric analyzer (Germany) at the Kazakhstan-Japan Innovation Center. It is proved that the increase in the concentration of cadmium, lead, mercury and arsenic in the daily diet of cows in winter or summer pasture periods is proportional to the increase in their content in milk. It was found that the level of heavy metals (Cd, Pb, As, Hg) in feed and milk does not exceed the MPC. The results obtained can serve as a basis for monitoring the state of the environment on the content of heavy metals in natural objects of the country.

Heavy metal uptake by plants from wastewater of different pulp concentrations and contaminated soils

There is a lack of information on the extent of mineral extraction by plants from wastewater in arid and semi-arid environments in developing countries. This research assesses the performance of Alhagi and Mallow plant species for the absorption of heavy metals around the tailings dam of a copper mine in Iran. The industrial wastewater, known as the pulp, from the copper mine site has different concentrations of heavy metals. In a laboratory setting the plants were cultivated and irrigated with different pulp concentrations (0, 20, 40, 60, 80, and 100%) in water. Heavy metals (chromium, manganese, cadmium, and lead) accumulation in the aerial parts and roots of the plants, and the in-situ soil around the tailings dam, were measured using an atomic absorption spectrometer. Soil mapping based on geostatistical methods was also conducted to show the spatial pattern of the contaminants in the surrounds of the tailings dam. Our results demonstrated that heavy metals accumulation was higher in the roots than in the aerial parts, and chromium and cadmium uptake by the Alhagi plant was higher than by the Mallow. The soil mapping showed that cadmium, lead and manganese had a higher dispersion compared to chromium, a high percentage being concentrated in some hotspots. Alhagi and Mallow plants, therefore, have a high potential to remediate heavy metal contaminated areas around the tailings dam and are recommended for widespread use. However, frequent plant harvesting should be encouraged to help reduce the migration of heavy metals from the contaminated soils into the surrounding environment. This research contributes to cleaner industrial production in non-developing and developing countries.

Sources and migration of heavy metals in a karst water system under the threats of an abandoned Pb–Zn mine, Southwest China

Water quality is of great concern under the threats of an abandoned Pb-Zn mine within a vulnerable karst area. This study focuses on tracing the sources, migration and relevant geochemical controls of heavy metals along with surface- and groundwater flow, and assessing their hazards posed to the environment by indexes of heavy metal pollution (HPI) and ecological risks (ERI). We analyzed the concentrations of lead (Pb), zinc (Zn), cadmium (Cd) and copper (Cu) in filtered and digested surface water (SW) and groundwater (GW) samples. Whilst sulfate concentrations, δ34SSO4 isotope, and other physical-chemical parameters were measured in filtered SW and GW. Clearly elevated concentrations of Zn and Cd have been observed in the upstream of the Sidi River (max. 1540.3/1676.7, and 19.9/19.4μg/L, for filtered/digested contents, respectively), and that of digested Pb and Cu (max. 17.8 and 114.6μg/L, respectively) in the downstream karst groundwater. Zn and Cd are likely introduced by the Sidi River which is found mostly in the dissolved phase. While Pb and Cu are sourced from the remobilization of upper lying polluted soil which mostly existed in the particulate phase. Zn and Cd concentrations in the karst groundwater peak at the losing section while Pb and Cu in the river peaks at the gaining section as a result of metals transport with SW-GW exchange. Dilution significantly decreases total metals concentration in the non-karst area. Due to the buffer effect caused by carbonates dissolution and bacterial sulfate reduction, a steady neutral pH can reduce the concentrations of the dissolved metals in the karst area. The ERI and HPI assessments show an acceptable level for surface- and groundwater. A long-term observation on the contents of undissolved metals needs to be conducted in karst areas which are threatened by metal(s) mines.

Migration and Transformation of Heavy Metals in the Soil of the Water-Level Fluctuation Zone in the Three Gorges Reservoir under Simulated Nitrogen Deposition

The accumulation of heavy metals (HMs) in the water-level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR) area is potentially harmful to the water environment. In order to reveal whether nitrogen (N) deposition is a potential driving factor for the migration and transformation of HMs (Cd, Cr, Cu, Ni, and Pb), a simulated N deposition experiment was performed on the soil in the WFLZ of the TGR. The results showed that the accumulative release amounts of HMs increased with the increase of N deposition. It was found that the Elovich equation, double-constant equation, and parabolic diffusion equation could well describe the release process of Cu, Cd, Cr, and Ni, while the double-constant equation, parabolic diffusion equation, and first-order equation could be applicable for Pb. The exchangeable fractions of HMs increased to varying degrees after the N deposition treatment, wherein Ni was most significant, indicating that N deposition could increase the ecological risk of HM pollution in the TGR area. The results provide insight into the major factors affecting the release of different HMs under N deposition in this vulnerable region ecologically.

Determination of phytotoxicity of wastes of mechanical processing of polymer mixtures with their mechanical activation

Biotesting to assess the complex relationships between soil, contaminants and living organisms and to determine the impact on ecosystems of pollutants whose concentrations may be beyond the sensitivity of the methods of determination and the composition of which cannot be determined. The aim is to establish the level of phytoxicity of wastes of mechanical processing of polymer mixtures, which are extracted from solid household waste with the help of indicator plants. Methods. Evaluation of phytotoxic effects was performed by vegetation methods according to ISO: 17402-2008, 17126-2005a, 11269-1: 2012a, 11269-2: 2012b. Contamination levels of the test samples were assessed according to ISO 16198.Objects of research: watercress seeds, mustard, wheat, corn, soybeans, barley and sludge from the processing of polymer mixtures composed of polymer particles of different composition and quantity, along with the remnants of paper labels. Studies of the content of heavy metals (Pb, Cu, Cd, Zn, Ni) were performed by atomic absorption method on a spectrophotometer type CAS-120.1 in accordance with generally accepted methods. Results. The studied sludge does not negatively affect the process of germination of watercress, mustard, corn. The level of phytotoxic effect of sludge on the germination of soybeans and barley was within acceptable limits and did not exceed 3.33%. The study of migration of heavy metals present in the sludge and their accumulation during the growth and development of plants showed their insignificant content regardless of the sludge content. Only the lead content was higher than in the control 2.13 times and 1.83 times higher than in "relatively clean soil". Conclusions. Determination of phytotoxicity of this type of sludge on the process of germination and vegetation changes of plants should be carried out on the most sensitive crops - watercress and mustard. A study of the migration of heavy metals present in the sludge and their accumulation during plant growth showed the presence of bound forms of heavy metals copper, nickel, cadmium and zinc - the migration of which has not been established. However, mobile forms of lead that migrate from the sludge and accumulate in plants have been identified.

Evaluating the adsorption of Shanghai silty clay to Cd(II), Pb(II), As(V), and Cr(VI): kinetic, equilibrium, and thermodynamic studies.

The adsorption properties of Shanghai silty clay (SSC) towards heavy metal ions Cd(II), Pb(II), As(V), and Cr(VI) were investigated by batch experiments. The effects of solid-solution ratio, pH, temperature, reaction time, and metal concentration on sorption were analyzed. In order to better understand the adsorption mechanisms, X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray fluorescence (XRF) were used to analyze the soil specimen before and after sorption. Three adsorption kinetic models and three adsorption isotherm models were used to analyze the adsorption characteristics. Thermodynamic parameters including changes in the Gibbs free energy (ΔG0), enthalpy (ΔH0), and entropy (ΔS0) were also calculated. Sorption capacity of SSC was compared with other clay minerals reported in the literatures. The results show that the selectivity sequence is Pb(II) > Cd(II) > As(V) > Cr(VI), with equilibrium sorption capacities of 26.46, 8.90, 2.80, and 1.85mgg-1, respectively. Adsorption is largely effective on the clay surface rather than on the crystals. The clay surface turns to be flat and slippery after adsorption. The Langmuir model shows the best fit for Cd(II) and Pb(II) data, while Freundlich model is more suitable for As(V) and Cr(VI). The optimum solid-solution ratios for sorption of Cd(II), Pb(II), As(V), and Cr(VI) are 15, 6, 40, and 40g L-1, respectively. The optimum pHs for Cd(II), Pb(II), As(V), and Cr(VI) adsorption are 9.0, 6.0, 7.0, and 2.0, respectively. The pseudo-second-order kinetic is found to be the dominant sorption mechanism of these four ions on SSC. For Cd(II) and Pb(II), both particle diffusion and film diffusion are rate-limiting factors, whereas for As(V) and Cr(VI), intraparticle diffusion is the rate-controlling factor. The thermodynamic analysis reveals that the adsorption of Cd(II) and Pb(II) is spontaneous and endothermic and the system disorder increases, while adsorption of As(V) and Cr(VI) is exothermic and the system disorder decreases. Compared with most clay minerals, natural SSC exhibits comparable adsorption capacity and thus can potentially be used as a landfill liner material to retard the migration of heavy metals.

Enhanced adsorption of heavy metals in groundwater using sand columns enriched with graphene oxide: Lab-scale experiments and process modeling

In this study, sand enriched with graphene oxide (GO) was applied as the medium to prevent heavy metals migration to groundwater. The retention of heavy metals including Cr(VI), As(III), Cd(II), and Pb(II) in the prepared medium was investigated. Batch experiments results revealed that the adsorption capacities of GO were 530.85, 170.1, 49.78, and 14.41 mg g−1 for Pb(II), Cd(II), As(III), and Cr(VI). The adsorption characteristics of GO were investigated by four linear and nonlinear isotherm models. Nonlinear Freundlich was the best model fitting the experimental observations in equilibrium reactions. The presence of GO enhanced adsorption capacity of the sand column to 92, 87, 88, and 94 % for Pb(II), Cd(II), As(III), and Cr(VI), respectively. The retention of single component and a mixture of heavy metals in the prepared medium was studied, showing that in the mixture of metals, the adsorption capacity of GO reduced by 14, 21, and 39 % for Pb(II), Cd(II), and As(III), respectively. A longer breakthrough time for GO was achieved for Pb(II) (10.83 h) and Cd(II) (4.23 h), indicating higher efficiency of the reactive medium in the retention of Pb(II) and Cd(II). Thomas and Adam’s–Bohart kinetic models were applied to express the dynamic characteristics of the proposed medium. Breakthrough curves of Cr(VI), Cd(II), and Pb(II)were successfully predicted by Thomas model, while that of As(III) was described better by Adam’s–Bohart model. This can be associated with the fact that the Thomas model assumes Langmuir isotherm, while adsorption behavior of As(III) was well-depicted by Freundlich isotherm.

Study on Rule of Heavy Metal Cu2+ in Small and Medium-Sized Tailings Ponds under Rainfall-Evaporation-Transpiration Coupling

In order to research the heavy metal migration rule of small and medium-sized tailings ponds under the action of climate, vegetation and groundwater, this paper consider the rainfall-evaporation-transpiration and coupling effect of groundwater, establish the model of different dam height, slope ratio, permeability coefficient and migration time impact on the migration of heavy metals of tailings dam, applied to Geostudio software, based on the orthogonal design method of factors significant difference analysis and design method of the comprehensive factors analysis, make up the simulation of the tailings heavy metal pollutants concentration of Cu2+ migration law. The results show that the height of tailing dam is inversely proportional to the average migration velocity of pollutant Cu2+. The effect of slope ratio on migration concentration of Cu2+ pollutant in tailings pond is not significant. The permeability coefficient has a highly significant influence on the migration of heavy metal Cu2+. At the same depth of tailings pond, when Cu2+ migrated for 365 days, the permeability coefficient increased with the increase of pollutant Cu2+ concentration. The concentration of heavy metal increased with the increase of migration time.

ИССЛЕДОВАНИЕ ВЛИЯНИЯ ТЯЖЕЛЫХ МЕТАЛЛОВ НА ПОЧВЕННЫЙ ПОКРОВ И МЕТОДЫ ИХ БИОРЕМЕДИАЦИОННОГО КОНТРОЛЯ

The article provides information on literature and patent information on the analysis of heavy metals formed during the extraction and processing of various types of raw materials for the production of non-ferrous metals and chemical products. The influence of heavy metals on the environment and living organisms, fauna and flora, as well as people living in different industrial regions of Kazakhstan is shown. Migration of heavy metals in the soil cover hydrosphere and atmosphere is shown. Based on the analytical review, the goals and objectives of the study are outlined, the chemical composition of raw materials for the main heavy metals formed in solid and dust - like production waste, and for technological processing in the production of lead and zinc in the East Kazakhstan and Turkestan regions is shown. Information is provided on the presence of heavy metals in the soil and their purification by various bioremediation methods. The data of soil analysis on the territory of the Turkestan region in recreation areas of people and in non-ferrous metallurgy enterprises nearby to the mining complex are presented. The dependence of the distribution of heavy metals and the chemical composition of the analyzed samples taken in various points of the Turkestan region and the city of Shymkent is shown. Data on the maximum content of heavy metals in the soil at which experimental worms die are established.

Assessment of Heavy Metals Migration in the Water - Soil - Fodder - Milk Food Chain in the Almaty Region

Assessment of Heavy Metals Migration in the Water - Soil - Fodder - Milk Food Chain in the Almaty Region

Determination of background values and assessment of pollution and ecological risk of heavy metals in urban agricultural soils of Yaoundé, Cameroon

Presence of heavy metals in urban agriculture is considered a major ecological risk. The aim of this research is to determine concentrations and background values of heavy metals in urban agricultural soils in Yaounde (Cameroon), in order to assess the degree of soil pollution and to evaluate ecological risk. Forty-five composite soil samples were taken from three market garden sites. Heavy metals (Cd, Cr, Cu, Mn, Ni, Pb and Zn) were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES). Geochemical background threshold values (GBTV) were determined using the robust statistical method of median ± median absolute deviation (MAD). Single (geoaccumulation index and enrichment factor) and complex (pollution load index and Nemerow pollution index) indices of pollution and of ecological risk (potential ecological risk index) were calculated. Significant difference between the three market garden sites was evaluated using the nonparametric test of Kruskal–Wallis. Median concentrations varied considerably, ranging from the highest to the lowest levels (mg/kg) as Mn 259.0 > Cu 111.2 > Zn 81.9 > Cr 54.6 > Pb 19.3 > Ni 14.9 > Cd 0.1. All median values were below their corresponding geochemical backgrounds, except for Pb. Single pollution indices showed that the study site was contaminated with Pb. Nemerow pollution index showed that 60% of soil samples were in the very high level of pollution. Based on potential ecological risk index, 16% of soil samples were at high risk. The three market garden sites differed significantly regarding their heavy metal contents, except Cd, as well as their pollution and ecological risk indices with the Nkolondom site having, in general, the highest levels. More than half of the study area is polluted, mainly with Pb, whereas ecological risk is limited to a small part of it. This study can be used as a baseline to develop future long-term risk assessment strategies on the potential migration of heavy metals in urban agricultural soils to vegetables, animals and human beings.

Uptake of Pb(II) onto microplastic-associated biofilms in freshwater: Adsorption and combined toxicity in comparison to natural solid substrates

In the present study, microplastic-associated biofilms were cultivated in an urban lake and a reservoir using virgin expanded polystyrene (PS). The uptake of Pb(II) onto both natural (suspended particles and surficial sediment) and anthropogenic (virgin microplastics and microplastic-associated biofilms) solid substrates was investigated and compared as a function of contact time, pH, and ionic strength in batch adsorption experiments. The adsorption isotherms revealed that biofilms enhanced the adsorption capacity of Pb(II) onto microplastics; however, natural substrates still exhibited a higher capacity. Ionic strength and pH significantly influenced the adsorption of Pb(II) onto all of the solid substrates. Under neutral conditions, competitive adsorption of Pb(II) was observed between anthropogenic solid substrates and natural substrates, which may further alter the distribution of Pb(II) among these solid substrates. The combined toxicity tests of Pb(II) and each solid substrate were carried out using Daphnia magna, the results indicated biofilm enhanced the combined toxicity of Pb(II) and microplastics. Therefore, biofilms not only intensified the vector role of microplastics in the migration of heavy metals in freshwater, but also enhanced their combined toxicity, which may have further potential ecological risks to freshwater ecosystems.