Individual and joint toxicity effects of Cu, Cr(III), and Cr(VI) on pakchoi: a comparison between solution and soil cultures

Characterization and application of Fe-modified biochar alleviating Cr(VI) stress in pak choi seedling cultivated in Cr-polluted hydroponics

Chromium (Cr) is one of the common environmental pollutants, which causes severe health hazards on human health and environmental security. In this study, we characterized two biochars, a raw biochar (RBC) and a Fe-modified biochar (MBC) made from poplar wood chips and determined the effect of the two biochars on remediation of hexavalent chromium (Cr(VI)) in hydroponic system by monitoring Pak choi growth. Results showed the surface area, pore number and pore volume were significantly higher in MBC than in PBC, but the pore size was larger in PBC than in MBC. When compared to the control, low concentrations of Cr(VI) (≤2 mg L-1) promoted the growth and biomass production of Pak choi by 10-78%. In contrast, the high concentrations of Cr(VI) (≥4 mg L-1) showed a significantly reduction of the growth and biomass production of Pak choi by 10-28%. Fe-modified biochar (MBC) had a more significant impact than RBC on the remediation of Cr in the Cr(VI) pollution and improved growth and biomass production of Pak choi to a greater extent. Our study indicated that MBC has a better effect on degrading Cr(VI) pollution. The findings provide scientific basis and reference for the remediation of heavy metals in aquatic ecosystems by using biochar.

Assessment soil cadmium and copper toxicity on barley growth and the influencing soil properties in subtropical agricultural soils

Evaluation joint cadmium (Cd) and copper (Cu) phytotoxicity in wide range of subtropical agricultural soils is highly vital for phytoremediation of soils contaminated with Cd and Cu. In this study, barley root elongation assays were performed in 30 representative soils in response to single and combined Cd and Cu inhibition. The single Cd caused nearly 50% inhibition of barley root elongation, and Cu induced more than 50% inhibition in most soils. Mixed Cd + Cu caused significant inhibition on barley growth with average relative root elongation values of 20.0% and 30.4% in soil with a pH < 7 and pH > 7, respectively. An antagonistic interaction was evaluated in combined Cd + Cu toxicity, which was strong in soils containing low soluble Cu and Cd contents. Soil pH was the controlling factor in predicting single and mixed Cd and Cu phytotoxicity, which could explain 44% and 46% variation of single Cd and Cu toxicity, respectively. Soil organic carbon and effective cation exchange capacity were another important factor positively influencing metal toxicity, which further improved empirical prediction models accuracy, with determined coefficient (r²) values of 0.44-0.84. These results provide a theoretical basis for soils Cd and Cu pollution control.

Metal(loid) speciation and transformation by aerobic methanotrophs

Manufacturing and resource industries are the key drivers for economic growth with a huge environmental cost (e.g. discharge of industrial effluents and post-mining substrates). Pollutants from waste streams, either organic or inorganic (e.g. heavy metals), are prone to interact with their physical environment that not only affects the ecosystem health but also the livelihood of local communities. Unlike organic pollutants, heavy metals or trace metals (e.g. chromium, mercury) are non-biodegradable, bioaccumulate through food-web interactions and are likely to have a long-term impact on ecosystem health. Microorganisms provide varied ecosystem services including climate regulation, purification of groundwater, rehabilitation of contaminated sites by detoxifying pollutants. Recent studies have highlighted the potential of methanotrophs, a group of bacteria that can use methane as a sole carbon and energy source, to transform toxic metal (loids) such as chromium, mercury and selenium. In this review, we synthesise recent advances in the role of essential metals (e.g. copper) for methanotroph activity, uptake mechanisms alongside their potential to transform toxic heavy metal (loids). Case studies are presented on chromium, selenium and mercury pollution from the tanneries, coal burning and artisanal gold mining, respectively, which are particular problems in the developing economy that we propose may be suitable for remediation by methanotrophs. Video Abstract.

Extended biotic ligand model for predicting combined Cu-Zn toxicity to wheat (Triticum aestivum L.): Incorporating the effects of concentration ratio, major cations and pH

Current risk assessment models for metals such as the biotic ligand model (BLM) are usually applied to individual metals, yet toxic metals are rarely found singly in the environment. In the present research, the toxicity of Cu and Zn alone and together were studied in wheat (Triticum aestivum L.) using different Ca²⁺ and Mg²⁺ concentrations, pH levels and Zn:Cu concentration ratios. The aim of the study was to better understand the toxicity effects of these two metals using BLMs and toxic units (TUs) from single and combined metal toxicity data. The results of single-metal toxicity tests showed that toxicity of Cu and Zn tended to decrease with increasing Ca²⁺ or Mg²⁺ concentrations, and that the effects of pH on Cu and Zn toxicity were related not only to free Cu²⁺ and Zn²⁺ activity, respectively, but also to other inorganic metal complex species. For the metal mixture, Cu-Zn interactions based on free ion activities were primarily additive for the different Ca²⁺ and Mg²⁺ concentrations and levels of pH. The toxicity data of individual metals derived by the BLM, which incorporated Ca²⁺ and Mg²⁺ competition and toxicity of inorganic metal complexes in a single-metal toxicity assessment, could predict the combined toxicity as a function of TU. There was good performance between the predicted and observed effects (root mean square error [RMSE] = 7.15, R² = 0.97) compared to that using a TU method with a model based on free ion activity (RMSE = 14.29, R² = 0.86). The overall findings indicated that bioavailability models that include those biochemistry processes may accurately predict the toxicity of metal mixtures.

Interactions and Toxicity of Cu-Zn mixtures to Hordeum vulgare in Different Soils Can Be Rationalized with Bioavailability-Based Prediction Models

Soil contamination with copper (Cu) is often associated with zinc (Zn), and the biological response to such mixed contamination is complex. Here, we investigated Cu and Zn mixture toxicity to Hordeum vulgare in three different soils, the premise being that the observed interactions are mainly due to effects on bioavailability. The toxic effect of Cu and Zn mixtures on seedling root elongation was more than additive (i.e., synergism) in soils with high and medium cation-exchange capacity (CEC) but less than additive (antagonism) in a low-CEC soil. This was found when we expressed the dose as the conventional total soil concentration. In contrast, antagonism was found in all soils when we expressed the dose as free-ion activities in soil solution, indicating that there is metal-ion competition for binding to the plant roots. Neither a concentration addition nor an independent action model explained mixture effects, irrespective of the dose expressions. In contrast, a multimetal BLM model and a WHAM-Ftox model successfully explained the mixture effects across all soils and showed that bioavailability factors mainly explain the interactions in soils. The WHAM-Ftox model is a promising tool for the risk assessment of mixed-metal contamination in soils.

Effects of Cu(II) on the Adsorption Behaviors of Cr(III) and Cr(VI) onto Kaolin

The adsorption of Cr(III) or Cr(VI) in the absence and presence of Cu(II) onto kaolin was investigated under pH 2.0–7.0. Results indicated that the adsorption rate was not necessarily proportional to the adsorption capacity. The solutions’ pH values played a key role in kaolin zeta potential(ζ), especially the hydrolysis behavior and saturation index of heavy metal ions. In the presence of Cu(II),qmixCr(III)reached the maximum adsorption capacity of 0.73 mg·g−1at pH 6.0, while the maximum adsorption capacity for the mixed Cr(VI) and Cu(II) system (qmixCr(VI)) was observed at pH 2.0 (0.38 mg·g−1). Comparing the adsorption behaviors and mechanisms, we found that kaolin prefers to adsorb hydrolyzed products of Cr(III) instead of Cr3+ion, while adsorption sites of kaolin surface were occupied primarily by Cu(II) through surface complexation, leading to Cu(II) inhibited Cr(VI) adsorption. Moreover, Cr(III) and Cr(VI) removal efficiency had a positive correlation with distribution coefficientKd. Cr(III) and Cr(VI) removal efficiency had a positive correlation with distribution coefficientKdand that of adsorption affinities of Cr(III) or Cr(VI) on kaolin was found to beKdCr(III) <KdCr(III)-Cu(II) andKdCr(VI) >KdCr(VI)-Cu(II).

Joint toxicity of heavy metals and chlorobenzenes to pyriformis Tetrahymena

Chlorobenzens and heavy metals are frequently detected in the environment, but few studies have assessed the joint toxicity of organic and inorganic contaminants. The joint toxicity of heavy metals and chlorobenzenes was evaluated in the present study. Growth metabolism of the joint toxicity was studied by microcalorimetry at 28°C, the growth constant (k) and inhibitory ratio (I) were calculated. Toxic unit (TU) and additional index (AI) were introduced to determine the outcome in combined tests, and the coexistence of Cu, Cd, Cr(III) and p-chlorobenzene was antagonism, and the effect of Cu, Cd, Cr(III) and o-chlorobenzene, Cu and 1,2,4-trichlorobenzene were synergism. In addition, micro-situation of the cell membrane surface of pyriformis Tetrahymena was observed by SEM. The cells suffered serious damage after sufficient acting time. ATR-FTIR spectra revealed that amide groups and PO2(-) of the phospholipid phospho-diester, both in the hydrophobic end exposed to the outer layer, were the easiest to be damaged.

Nanosized metal oxide and nanobelts prepared by selective dealloying of Ti-based amorphous powders

Two typical nanomaterials, nanosized metal oxides and nanobelts, are obtained in one-pot selective dealloying process by using multiple-component Ti-based amorphous powders as dealloying precursors. The microstructure and photoelectric conversion property of the as-synthesized Zr-doped nanobelts are comprehensively investigated. Particularly, a core-shell structure, for example, residual amorphous alloy as the microsized core and nanosized metal oxide composites (mainly TiO2 and CuO) as the shell, forms as a byproduct of the selective dealloying. These resultant metal oxide composites show large specific surface area, and superior adsorption efficiency and capacity for removing toxic Cr(6+) in aqueous solution. The differences in the standard electrode potentials between the multiple-component elements in amorphous powders trigger their selective dealloying in alkaline solutions.