Cd Species Research Articles

Cadmium behavior in sulfur-bearing aqueous environments: insight from CdS solubility measurements at 25-80°C

The present experimental study explores the effects of temperature and sulfur in Cd aqueous geochemistry under reduced conditions. Greenockite CdS solubility is measured in H2O-H2S-HClO4-NaHS solutions at 25-80°C as a function of pH and sulfur concentration. Based on solubility product measurements in highly acid solutions, the standard thermodynamic properties of greenockite (CdS) are revised, and the recommended value of ∆fG0298.15 for greenockite CdS(s) is -151.5±0.3 kJ mol-1. The stability of greenockite (CdS) is higher than predicted by calculations using previous literature data. At 80°C, the stability constants for Cd-HS complexes are measured for the first time, the values are 10–5.65±1.00 for CdS(s) + H+ = CdHS+, 10–6.00±0.40 for CdS(s) + H2S0(aq) = Cd(HS)20(aq), 10–3.87±0.10 for CdS(s) + H2S0(aq) + HS- = Cd(HS)3-, and 10–3.53±0.20 for CdS(s) + H2S0(aq) + 2HS- = Cd(HS)42-. Modeling of Cd behavior at 3-200°C shows that Cd-HS species are more important than previously believed. The fraction of Cd(HS)n2-n (n = 1-4) complexes increases with mH2S and decreases with T. Thus, in euxinic marine environments with mH2S ≥ 10-5, Cd speciation changes from Cd-Cl to Cd-HS. This speciation change is expected to affect Cd isotope fractionation and should be accounted for when applying Cd isotopic signature as a paleo tracer in marine sediments. The new thermodynamic data are indispensable for modeling Cd behavior in response to pH, T, and mH2S. As a function of these parameters, sulfur has the main control on Cd geochemistry being the main factor of Cd precipitation at low mH2S and favoring Cd mobilization at high mH2S.

Effect of Arbuscular Mycorrhiza Fungus Diversispora eburnea Inoculation on Lolium perenne and Amorpha fruticosa Growth, Cadmium Uptake, and Soil Cadmium Speciation in Cadmium-Contaminated Soil

Cadmium (Cd) pollution has become aggravated during the past decades of industrialization, severely endangering human health through its entry into the food chain. While it is well understood that arbuscular mycorrhizal fungi (AMF) have a strong ability to regulate plant growth and Cd uptake, studies investigating how they affect soil Cd speciation and influence Cd uptake are limited. We designed a pot experiment comprising two AMF-inoculant groups (inoculation with Diversispora eburnea or no inoculation), three Cd concentration levels (0, 5, and 15 mg/kg), and two plant species (Lolium perenne and Amorpha fruticosa) to study the effect of AMF Diversispora eburnea on plant growth, Cd uptake, and Cd speciation in the soil. The results revealed that L. perenne exhibited higher productivity and greater Cd uptake than A. fruticosa, regardless of AMF D. eburnea inoculation. However, AMF D. eburnea significantly altered soil Cd speciation by increasing the proportion of exchangeable Cd and decreasing residual Cd, resulting in Cd enrichment in the plant root organs and the elimination of Cd from the polluted soils. Our experiments demonstrate that inoculating plants with AMF D. eburnea is an effective alternative strategy for remediating Cd-contaminated soil.

Synthesis of fungal chitosan-polystyrene modified by nanoparticles of binary metals for the removal of heavy metals from waste aqueous media.

The objective of this study was to assess the efficacy of fungal chitosan-polystyrene-Co-nanocomposites (FCPNC) as a material for the adsorptive removal of cadmium (Cd) ions from aqueous solutions. The synthesis and characterization of FCPNC were accomplished using various analytical techniques, including Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) analysis, and dynamic light scattering (DLS). The effectiveness of this adsorbent in removing Cd(ii) species from solution matrices was systematically investigated, resulting in the achievement of a maximum adsorption capacity of approximately 112.36 mg g-1. This high adsorption capacity was detected using the following operational parameters: solution pH equals 5.0, 60 min as a contact time between the adsorbent and Cd(ii) solution, Cd initial concentration of 50 ppm, adsorbent dosage of 0.5 g L-1 and room temperature. The process of cadmium adsorption by FCPNC was found to follow the Langmuir isotherm model, suggesting that a chemical reaction occurs on the biosorbent surface. Kinetic studies have demonstrated that the cadmium removal process aligns well with the pseudo-second-order model. The thermodynamic analysis revealed the following values: ΔH° = 25.89 kJ mol-1, ΔG° = -21.58 kJ mol-1, and ΔS° = 159.30 J mol-1 K-1. These values indicate that the sorption process is endothermic, spontaneous, and feasible. These findings suggest the potential of FCPNC as an exceptionally effective biosorbent for the removal of water contaminants.

Polyethylene and poly (butyleneadipate-co-terephthalate)-based biodegradable microplastics modulate the bioavailability and speciation of Cd and As in soil: Insights into transformation mechanisms

Microplastics (MPs) that enter the soil can alter the physicochemical and biochemical properties of soil and affect speciation of heavy metals (HMs), thereby perturbing the bioavailability of HMs. However, the mechanisms underlying these effects are not understood. Therefore, we investigated the effects of MPs from poly (butyleneadipate-co-terephthalate)-based biodegradable mulch (BM) and polyethylene mulch (PM) in Cd- or As-contaminated soil on soil properties and speciation of HMs. MPs were characterised using Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The addition of MPs reduced the bioavailability of HMs in soil and promoted the transformation of HMs into inert fractions. The mechanisms underlying the reduction of the bioavailability of HMs in soils could be as follows: (1) the entry of MPs into the soil changed its properties, which reduced the bioavailability of HMs; (2) FTIR and XPS analyses revealed that the hydroxyl and carboxyl groups and benzene ring present on the surface of aged MPs stabilized complexes (As(V)-O) with As(V) may have directly reduced the bioavailability of As(V) in soil; (3) aged BM exposed more amounts and types of reactive functional groups and was more effective in stabilising soil HMs than PM. Overall, this study provides new insights regarding the complexation mechanisms of soil HMs by MPs from different plastic mulch sources.

Adsorption of cadmium vapor by calcium-based adsorbents in simulated flue gas: Experimental and density functional theory studies

Calcium-based adsorbents are considered as effective high-temperature adsorbents for the removal of heavy metal in the furnace. In this work, the adsorption characteristics and reaction mechanism of calcium-based adsorbents for cadmium vapor were investigated by adsorption experiments and density functional theory (DFT) methods. The adsorption experiments showed that the adsorption performance of CaO for cadmium vapor was significantly better than Ca(OH)2, Calcite and CaSO4. The adsorption capacity of CaO adsorbent for cadmium increased firstly and then decreased with the increase of temperature. The enhanced trapping capacity of CaO for cadmium vapor with increasing temperature was associated with enhanced chemisorption, while the decrease in trapping capacity at high temperature could be attributed to adsorbent sintering. In addition, the results of DFT calculations showed that CaO had good adsorption capacity for various Cd species, and the order of the adsorption capacity was: CdO > CdCl2 > CdCl > Cd. The chemisorption mechanism dominated the adsorption of Cd species on the CaO (001) surface, and the central O site was the reactive site and electron transfer channel for the adsorption of Cd. This work provided a reference for the application of heavy metal vapor capture technology based on calcium-based minerals.

Effects of short-range order on phase equilibria and opto-electronic properties of ternary alloy ZnxCd1-xTe

We employ first principles methods based on density functional theory and beyond to study ZnxCd1-xTe, 0≤x≤1, alloys in the zinc blende (B3) crystal structure. Cluster Expansion and Monte Carlo formalisms were deployed to provide a phase diagram determining consolute temperature of 387 K at 40% Zn concentration. Opto-electronic properties are computed with the hybrid HSE06 functional for disordered ZnxCd1-xTe alloys, which were simulated using special quasi-random structures. Bowing effects in the bandgap and effective carrier masses were observed in alloying which can be attributed to local geometrical distortions as portrayed by bond length distributions. Downward bowing in the electronic bandgap will be beneficial in photovoltaic applications through increased net photocurrent. Absorption coefficients show robust absorption in ZnxCd1-xTe as indicated by substantial optical absorption throughout all Zn concentrations, aided by low reflectivity that ensures a high absorption. Presence of short-range order in Zn0.25Cd0.75Te was observed through clustering and anti-clustering among different cationic species of Zn and Cd. It has a value of 0.11 at 1000 K compared to −0.79 at 400 K. The bandgap of Zn0.25Cd0.75Te at 1000 K was found to be slightly higher, 0.05 eV, than at 400 K, consistent with the value of short-range order. Thus, short-range order and possibly composition can be used in the design and synthesis of the appropriate solar cell, thereby improving the performance in this system.

Microbial community characteristics of cadmium speciation transformation in soil after iron-based materials application

Although numerous studies on the stabilization of heavy metals in contaminated soil with iron-based materials have been conducted, the role of microorganisms in iron species and Cd speciation transformation has not been fully elucidated. Here, 2-line ferrihydrite (MGE) and goethite (GE) were prepared and used to stabilize Cd in contaminated soil. Based on the microbial community characteristics in the early, middle, and later stages, the role of microorganisms in Cd speciation transformation was explored. The exchangeable Cd decreased by 10.73 %–51.31 %, while the iron‑manganese oxide-bound speciation, and the residual speciation increased after MGE/GE application. The relative abundance of Proteobacteria, and Sphingomonas were obviously increased, and Trichoderma and Mortierella were decreased in the MGE group. The outcome in the GE group was the opposite. The complexity of the bacterial and fungal community network was increased in the MGE and GE groups. The lignin in MGE could stimulate the growth of iron-reducing bacteria (FeRB) and promote the reduction of iron minerals to Fe2+. The Fe2+ in the soil was oxidized and hydrolysed, which might lead to an increase in the soil pH. During the process of iron species transformation and iron redox mediated by FeRB, Cd was transformed to more stable speciation.

Experimental and theoretical studies on the adsorption characteristics of Si/Al-based adsorbents for lead and cadmium in incineration flue gas

Si/Al-based adsorbents are effective adsorbents for capturing heavy metals in incineration flue gases at high temperatures in the furnace. In this work, the adsorption characteristics and adsorption mechanisms of Si/Al-based adsorbents for lead and cadmium vapors were studied using a combination of experimental and density functional theory (DFT) calculations. The trapping performance of a series of Si/Al-based adsorbents for Pb and Cd vapors was investigated using a self-designed gas-solid two-phase rapid adsorption experimental system. The results showed that kaolinite and montmorillonite exhibited better heavy metal adsorption capacity than SiO2 and Al2O3, and were significantly stronger for Pb than for Cd. Chemisorption dominated the capture of Pb/Cd by Si/Al-based adsorbents at high temperatures. The results of DFT calculations indicated that the chemisorption mechanisms dominated the adsorption of Pb and Cd species on the metakaolinite (001) surface, and the adsorption energy of Pb species on the metakaolinite surface was greater than that of Cd species. The exposed O atoms and unsaturated Al atoms of metakaolinite (001) surface were effective adsorption active sites for heavy metals and their chlorides. In the adsorption reaction, the binding of Pb/Cd atoms and surface exposed O sites, as well as the strong interaction between Cl and unsaturated Al atoms, were responsible for the capture of Pb and Cd chlorides by metakaolinite.

Subcellular localization and chemical speciation of Cd in Arabidopsis halleri ssp. gemmifera to reveal its hyperaccumulating and detoxification strategies

The study involving low-level Cd using Arabidopsis halleri ssp. gemmifera is likely suitable for realistic Cd phytoremediation since the available Cd in the contaminated soil is considered low. However, the comprehensive evidence on how this plant undergoes hyperaccumulating-detoxification strategies under low Cd levels in its root, stem, and leaves remained questionable. Therefore, Cd uptake and distribution, intracellular localization, Cd speciation, and metal chelation compounds, glutathione (GSH) and phytochelatins (PCs), were assessed in a time-course analysis to portray their involvement in the Cd hyperaccumulating mechanisms. The results showed that under low Cd stress, 0.3 µmol/L of Cd was completely absorbed by this plant during 336 h, indicating a high Cd uptake system. At the same time, more Cd content accumulated in the stem and leaves compared to Cd content in the root. Excessive Cd being transported to the upper part was supported by the availability of more than 80% of Cd speciation categorized as mobile fractions (ethanol-and H 2 O-extractable forms). The highest Cd soluble fraction found in the root during initial and middle exposure promoted Cd intracellular translocation to stem and leaves. At the end of exposure (336 h), we predicted Cd was compartmentalized in the less metabolite sites, cell wall and vacuole due to less toxicity effect, which was also found in high Cd level (30 µmol/L). Furthermore, GSH and PC as metal complexation compounds owned a minor role even under high Cd exposure level. Therefore, regardless of the Cd supply level, elevating of metal transport and sequestration in the shoots were preferred over thiol complexed forms as common hyperaccumulating strategies in A. halleri ssp. gemmifera. • At low Cd supply, Arabidopsis halleri ssp. gemmifera does not accumulate in the root. • Cd is finally sequestrated in the leaves and existed mainly as soluble fraction. • More than 80 % of Cd chemical form in the tissues was mobile forms. • Glutathione and phytochelatins play a minor role in Cd chelating mechanisms.

In-situ immobilization mechanism of lead and cadmium species by pyrolytic char in MSW pyrolysis process: Effect of chlorine group

The release of heavy metals brings severe environmental pollution and affects product application during the pyrolysis of municipal solid waste. Due to the excellent immobilization properties, the utilization of pyrolytic char for heavy metals is a promising approach, yet the microscopic mechanism between Cl functional groups on the char surface and Pb/Cd species remains unclear. Thus, density functional theory was applied to study the interaction mechanism between them through geometric optimization, electronic structure, and chemical reaction. Owing to the strong valence electron hybridization, the Cl functional group greatly increases the adsorption stability of Pb and Cd species. In addition, the armchair edge of char exhibits better affinity to heavy metal (belonging to the chemisorption) than that of the zigzag edge, whose fixation stability follows the order of Pb0>PbCl2>CdCl2>Cd0. Compared with the chlorination by HCl, the Cl group raises the chlorination barrier of Pb0 and Cd0 over the zigzag edge, while there is no apparent tendency over the armchair edge, since it increases the energy barrier of Cd0 but lowers that of Pb0. Therefore, the Cl functional groups of char promote the fixation of Pb/Cd and inhibit the generation of the easily leachable CdCl2, thereby facilitating the in-situ immobilization of heavy metals.

Effects of CaCl2 on concentration and speciation of soil Cd around a Pb-Zn mine

Remediation of Cd contaminated soil around mines is still the current academic focus. In order to discuss the activation and extraction effects of CaCl2 on concentration and speciation of soil Cd around a Pb-Zn mine, field investigation and soil sample collection were conducted. Batch experiment was used to study the influence of concentration and time on Cd activation and extraction. Cd contents were analysed. And Cd species were extracted by sequential extraction procedures. Results show that there are obvious concentration effect and time effect for Cd extraction by CaCl2. The best choice of CaCl2 concentration is 0.1 mol/L. And the best extraction time is 4h. Results also show that the percentage and content of exchangeable Cd is the lowest with the value of 9.1% and 0.16 mg/kg when the CaCl2 content is 0.1 mol/L. And the percentage and content of exchangeable Cd is the lowest with the value of 10.5% and 0.18 mg/kg when the extraction time is 4h. It is found that transformation of Cd species such as exchangeable, carbonates bound and Fe-Mn oxides bound should be the main reasons of concentration effect and time effect. And the extraction of soil Cd around a Pb-Zn mine by CaCl2 is a complex coupling process of dissolution, ion exchange, desorption, acidolysis, oxidation-reduction and complexation, etc. All of these above will aid to provide a basis for the research, development and application of remediation technology for Cd contaminated soil around this and other Pb-Zn mines.

Effect of organic acids and soil particle size on heavy metal removal from bulk soil with washing.

To evaluate the effect of soil particle size on heavy metals removal by washing, two soil samples were collected around a lead-zinc mining area (SM) and lead-zinc smelter (SS). The total content of Cd, Pb and Zn in SM and SS were determined. And the effect of soil particle size on Cd removal by low molecular organic acids was studied. The results showed that Cd was the main pollutant and the total content of Cd in SS can reach to 24.8mg Kg-1. 68.4% of the total Cd in SM existed in the form of residual state, while 54.7% of the total Cd in SS was in weak acid extractable state. About 50.0% of the Cd distributed in < 2μm soil size fraction. The washing results indicated that citric acid was a highly efficient eluent among the five low molecular weight organic acids (citric acid, malic acid, tartaric acid, oxalic acid and acetic acid). After washing, 40% and 69.6% of the total Cd in SS and SM can be removed by citric acid, respectively. While only 18.7-40.2% and 32.6-68.7% of Cd was removed from different size fractions of SM and SS, respectively. The species of Cd in soil size fractions affected the removal effect of citric acid. The citric acid can easily remove the weak acid extractable and reducible form of Cd in soil. After eluted by citric acid, the bioavailability of Cd in soil decreased markedly, and the highest decreasing rate reached 93%.

Effects of Straw Return and Moisture Condition on Temporal Changes of DOM Composition and Cd Speciation in Polluted Farmland Soil.

Straw return can improve soil quality and change the mobility and bioavailability of pollutants in soil. Elevated cadmium (Cd) contents in farmland soils were often reported. However, the impacts of straw-derived dissolved organic matter (DOM) on Cd speciation in soil remain poorly understood. In this study, the effects of straw return and moisture condition on temporal changes of DOM composition and Cd speciation in farmland soils were explored through a laboratory incubation experiment. The humified components of DOM were negatively correlated with exchangeable, carbonate-bound, and Fe-Mn oxide-bound Cd (p < 0.01), while its protein-like component was negatively correlated with residual Cd (p < 0.01). It was found that selected fluorescence parameters could be used to predict temporal changes of Cd geochemical fractions. Straw addition led to increases in soil DOM content during the first three days of the incubation. Flooding should be avoided in the first three days following the straw application to reduce the risk of DOM-facilitated Cd mobilization.

Cadmium in the hyperaccumulating mushroom Thelephora penicillata: Intracellular speciation and isotopic composition

Thelephora penicillata is an ectomycorrhizal mushroom that can accumulate extraordinarily high concentrations of Cd, As, Cu, and Zn in its fruit-bodies. To better understand its element accumulation ability, we compared the element concentrations in T. penicillata with 10 distinct ectomycorrhizal mushroom species growing at the same site (Karlina Pila, Czech Republic). On average, T. penicillata accumulated 330, 2130, 26, and 4 times more Cd, As, Cu, and Zn, respectively, than other mushrooms. Size-exclusion chromatography and an electrophoretic analysis of T. penicillata cell extracts indicate that intracellular Cd may be present mainly in >1 kDa, presumably compartmentalized, Cd species, and partially binding with 6-kDa cysteinyl-containing peptide(s) resembling metallothioneins. The cadmium isotopic composition of mushroom fruit-bodies, soil digests, and soil extracts was investigated by thermal ionization mass spectrometry (TIMS) with double spike correction. The isotopic composition (δ114/110Cd) of ectomycorrhizal mushrooms from Karlina Pila varied in a wide range of −0.37 to +0.14 ‰. However, remarkably low δ114/110Cd values were observed in the majority of the investigated mushrooms when compared to the relatively homogeneous Cd isotopic composition of bulk soil (δ114/110Cd = +0.09 ‰) and the comparatively heavy isotopic composition of soil extracts (mean δ114/110Cd values of +0.11 ± 0.01 ‰ and +0.22 ± 0.01 ‰, depending on the extraction method). The isotopic composition of Cd hyperaccumulated in T. penicillata essentially matched the mycoavailable soil Cd fraction. However, most isotopic data indicates isotopic fractionation at the soil/fruit-body interface, which could be of environmental significance.

Interaction mechanism between cadmium species and Fe/Mn-doped carbon materials in municipal solid waste incineration flue gas: A density functional theory study

The interaction mechanism between cadmium (Cd) species and Fe/Mn-modified carbon materials is unveiled by density functional theory (DFT) to inspire the clean and efficient disposal of Cd pollutants in municipal solid waste (MSW) flue gas. The interaction of Cd pollutants over the surface mainly includes adsorption and catalysis. Regarding adsorption, the Fe/Mn-doped carbon martial can effectively improve the surface chemical activity and immobilization ability for Cd species, following the order of CdO > CdCl2 > Cd0. As for catalysis, the Fe/Mn doping promotes the accumulation of easy-to-trap CdO by elevating the CdO generation and suppressing the CdO conversion, and the conversion of difficult-to-trap Cd0 to the relatively easy-to-trap CdCl2 is also promoted. Herein, Mn doping exhibits better performance in both adsorption and catalysis than that of Fe. Consequently, the doping of Mn effectively synergizes the adsorption and catalysis on the surface, significantly improving the removal efficiency of Cd contaminants.

Speciation of Cu, Cd, Pb and Zn in a contaminated harbor and comparison to environmental quality standards

The water column of harbors contains significant amounts of (priority) hazardous trace metals that may be released into coastal areas of high societal and economic interests where they may disturb their fragile equilibria. To deepen our understanding of the processes that influence the transport of the various metal fractions and allow for a more rigorous environmental risk assessment, it is important to spatially monitor the relevant chemical speciation of these metals. It is of particular interest to assess their so-called dynamic fraction, which comprises the dissolved chemical forms that are potentially bioavailable to living organisms. In this study this was achieved in the Genoa Harbor (NW Italy) for copper (Cu), lead (Pb), cadmium (Cd) and zinc (Zn) by applying a multi-method approach. For the first time in this system the dynamic fractions of the target metals (CuDyn, CdDyn, PbDyn, ZnDyn) were observed in real-time on-board by voltammetry using innovative electrochemical sensing devices. Trace metals in the operationally defined dissolved <0.2μm and <0.02μm fractions were equally quantified through sampling/laboratory-based techniques. The obtained results showed a clear spatial trend for all studied metals from the enclosed contaminated part of the harbor towards the open part. The highest CuDyn and CdDyn fractions were found in the inner part of the harbor while the highest PbDyn fraction was found in the open part. The proportion of ZnDyn was negligible in the sampled area. Small and coarse colloids were involved in Cu, Cd and Zn partitioning while only coarse colloids played an important role in Pb partitioning. The determined concentrations were compared to the Environmental Quality Standards (EQS) established by the EU and those determined by the Australia and New Zealand to trigger for 99 and 95% species protection values. The results of this work allow us to highlight gaps in the EQS for which metal concentration thresholds are excessively high or non-existent and should urgently be revised. They also reflect the need to quantify the potentially bioavailable fraction of hazardous trace metals instead of just their total dissolved concentrations. The data support the establishment of environmental quality standards and guidelines based on realistic risk assessment to protect aquatic life and resources and ultimately human health.

Speciation and risk assessment of Zn, Pb, and Cd in bottom sediments of two small upland dam reservoirs, Poland

Sediments of two small dam reservoirs in Poland, Zalew Zemborzycki (ZZ) and Brody Iłżeckie (BI), were studied. Sediments from both reservoirs were sampled at 17 sites in the transects perpendicular to the shoreline, at the river inflow and the frontal dam and analysed using the BCR procedure for speciation of zinc (Zn), lead (Pb) and cadmium (Cd). The risk assessment code (RAC) and the individual contamination factor (ICF) were determined. In BI, the sediments were removed from the considerable part of the reservoir, creating an opportunity to study the effect of dredging on the speciation of trace metals. Trace metals partitioning was differentiated according to the transect/site and in the case of BI sediments also on the transect location in the dredged or undredged part of the reservoir. Considering ZZ sediments, the order of fractions for Zn, Pb and Cd according to a decreasing overall mean percentage contribution to total metal content was the same: F4 (residual) >F3 (oxidisable) >F2 (reducible) >F1 (acid soluble). In sediments of ZZ at most sites, the RAC for Zn, Pb and Cd revealed low or medium risk and ICF low or moderate contamination. For BI sediments, the order of fractions for Pb was similar while for Zn and Cd quite the opposite compared to the sediments of ZZ and it was: Pb–F4>F3>F1>F2, Zn–F1>F3>F2>F4, Cd–F1>F2>F3>F4. For BI sediments, RAC values for Zn, on average, indicated high and very high ecological risk; for Pb low and moderate risk and for Cd – high risk in the initial part and dredged part and according to the average value in the reservoir, while a medium risk in undredged part sediments. The ICF index showed high contamination with Cd for all BI sediments. The sequential analysis showed that Pb is poorly mobile as in sediments of both reservoirs residual fractions accounted, on average, for about 60% of the total content.

Long-term bioremediation of cadmium contaminated sediment using sulfate reducing bacteria: Perspective on different depths of the sediment profile

Sulfate reducing bacteria (SRB) can be utilized for the in-situ remediation of heavy metal contaminated sediment, and the remediation processes occurring at different depths of the sediment profile are critical to the practical application of SRB. In this study, column reactors with sediment profile sampling devices were designed to investigate the variations in Cd speciation, labile S2− and microbial community at different depths of the sediment profile during the bioremediation of Cd contaminated sediment by SRB, over a 270 day monitoring period. In order to evaluate the remediation effect, in vitro extraction of sediment was performed with Sipunculus nudus intestinal juice to determine Cd bioavailability. Results showed that bioremediation caused acid soluble Cd to decrease in the surface sediment (0–6 cm), while reducible and residual fractions of Cd increased. The bioavailability of Cd in the surface sediment reduced by 25.1 % compared to before bioremediation, while no significant changes were observed in other sediment layers. These results indicate that the surface sediment was the main remediation range when SRB were initially added at the water–sediment interface. Furthermore, labile Cd and S2− in the surface sediment of the bioremediation column reactor exhibited a negative correlation (P < 0.1), while they were positively correlated in the sediment profile below 12 cm (P < 0.1) and throughout the whole control column reactor (P < 0.1). This demonstrates that the correlation between labile Cd and S2− could potentially be used as an indicator to reflect the remediation performance. Principal component analysis results revealed that the remediation effect was related to the different Cd fractions, labile S2− and microbial community in the sediment, among which Cd speciation was the most influential factor. Overall, this research provides comprehensive foundation for the practical application of SRB for sediment in-situ remediation.

Effects of four endophytic bacteria on cadmium speciation and remediation efficiency of Sedum plumbizincicola in farmland soil.

Cadmium (Cd) pollution in farmland soils severely affects agricultural production safety, thereby threatening human health. Sedum plumbizincicola is a Cd and Zn hyperaccumulator commonly used for the phytoremediation of Cd-contaminated soil. This study was aimed to improve the remediation effect of S. plumbizincicola on Cd-contaminated farmland soil and provide a theoretical basis for the enhancement of endophytic bacteria in the repair of Cd-contaminated soil with S. plumbizincicola. Four kinds of endophytic bacteria, namely Buttiauxella, Pedobacter, Aeromonas eucrenophila, and Ralstonia pickettii, were used, and soil culture experiments and pot experiments were conducted to explore the effects of endophytic bacteria on soil Cd speciation and phytoremediation efficiency of Cd-contaminated farmland soils. Under the experimental conditions, after inoculation with endophytic bacteria, the soil pH was effectively reduced, content of weak acid-extracted Cd and oxidizable Cd increased, and content of reducible Cd and residual Cd decreased. Soil Cd activity was increased, and the availability coefficient of soil Cd increased by 1.15 to 6.41 units compared with that of the control (CK2). Compared with CK2, the biomass of S. plumbizincicola significantly increased by 23.23-55.12%; Cd content in shoots and roots of S. plumbizincicola increased by 29.63-46.01% and 11.42-84.47%, respectively; and bioconcentration factor was 2.13 to 2.72 times that of CK2. The Cd removal rate of S. plumbizincicola monocropping was 48.25%. When S. plumbizincicola was planted with inoculating endophytic bacteria, the Cd removal rate in the soil reached 61.18-71.49%, which was significantly higher than that of CK2 (p < 0.05). The treatment with endophytic bacteria activated soil Cd, promoted the growth of S. plumbizincicola, increased its Cd content, and enhanced the phytoremediation of Cd-contaminated farmland soil. Therefore, endophytic bacteria can be used to improve the remediation efficiency of S. plumbizincicola in Cd-contaminated farmland soils.

Enrichment and speciation changes of Cu and Cd in black soldier fly (Hermetia illucens) larval compost and their effects on larval growth performance

Municipal sludge (MS), rainwater sludge (RS), and kitchen waste (KW) were used as nutritional supplements for black soldier fly larvae (BSFL). Cd (52.3 %) was more easily assimilated in the BSFL body than Cu (34.8 %). After biotransformation in BSFL, the weak acid-soluble fraction (F1) of Cu and Cd increased by an average of 29.0 % and 42.7 %, respectively, whereas the reducible fraction (F2) of Cu and Cd decreased by an average of 13.8 % and 56.4 %, respectively, in the BSFL sand (BSFL feces and waste residues). A significant correlation (P < 0.01) was found between pH and the speciation of Cu and Cd. The abundance of Bacteroides had a positive correlation (P < 0.05) with the F1 of Cu, an extremely significant negative correlation (P < 0.001) with the F2 of Cd, and an extremely significant positive correlation with the F1 of Cd (P < 0.001). In addition, Cu and Cd exposures significantly (P < 0.01) reduced larval weight by 67.7 % and 45.3 %, respectively, pupation rate by 46.3 % and 26.5 %, respectively, and eclosion rate by 35.5 % and 33.4 %, respectively. Exposure to high concentrations of Cu and Cd also prolonged the development cycle (1–12 days) of BSFL and led to the failure of BSFL to complete their metamorphosis.