Potassium (K⁺) is the most abundant cation in phytoplankton cells, but its impact on Microcystis aeruginosa (M. aeruginosa) has not been fully documented. This study presents evidence of how K⁺ availability affects the growth, oxidative stress and microcystin (MC) production of M. aeruginosa. The iTRAQ-based proteomic analysis revealed that during K⁺ deficiency, serious oxidative damage occurred and the photosynthesis-associated and ABC transporter-related proteins in M. aeruginosa were substantially downregulated. In the absence of K⁺, a 69.26% reduction in cell density was shown, and both the photosynthesis and iron uptake were depressed, which triggered a declined production of ATP and expression of MC synthetases genes (mcyA, B and D), and MC exporters (mcyH). Through the impairment of both the MC biosynthesis and MC transportation out of cells, K⁺ depletion caused an 85.89% reduction of extracellular MC content at the end of the study. However, with increasing in the available K⁺ concentrations, photosynthesis efficiency, the expression of ABC-transporter proteins, and the transcription of mcy genes displayed slight differences compared with those in the control group. This work represents evidence that K⁺ availability can regulate the physiological metabolic activity of M. aeruginosa and K⁺ deficiency leads to depressed growth and MC production in M. aeruginosa.

Manganese oxides (MnOₓ) and Mn²⁺ usually co-exist in the natural environment, as well as in water treatments for Mn²⁺ removal. Therefore, it is necessary to investigate the influence of Mn²⁺ on the stability of MnOₓ nanoparticles, as it is vital to their fate and reactivity. In this study, we used the time-resolved dynamic light scattering technique to study the influence of Mn²⁺ on the initial aggregation kinetics of MnOₓ nanoparticles. The results show that Mn²⁺ was highly efficient in destabilizing MnOₓ nanoparticles. The critical coagulation concentration ratio of Mn²⁺ (0.3 mM) to Na⁺ (30 mM) was 2⁻⁶.⁶⁴, which is beyond the ratio range indicated by the Schulze-Hardy rule. This is due to the coordination bond formed between Mn²⁺ and the surface O of MnOₓ, which could efficiently decrease the negative surface charge of MnOₓ. As a result, in the co-presence of Mn²⁺ and Na⁺, a small amount of Mn²⁺ (5 μM) could efficiently neutralize the negative charge of MnOₓ, thereby decreasing the amount of Na⁺, which mainly destabilized nanoparticles through electric double-layer compression, required to initiate aggregation. Further, Mn²⁺ behaved as a cation bridge linking both the negatively charged MnOₓ and humic acid, thereby increasing the stability of the MnOₓ nanoparticles as a result of the steric repulsion of the adsorbed humic acid. The results of this study enhance the understanding of the stability of the MnOₓ nanoparticles in the natural environment, as well as in water treatments.

Understanding of cadmium (Cd) uptake mechanism and development of lower Cd crop genotypes are crucial for combating its phytotoxicity and meeting 70% increase in food demand by 2050. Bio-accumulation of Cd continuously challenges quality of life specifically in regions without adequate environmental planning. Here, we investigated the mechanisms operating in Cd tolerance of two rice genotypes (Heizhan-43 and Yinni-801). Damage to chlorophyll contents and PSII, histochemical staining and quantification of reactive oxygen species (ROS), cell viability and osmolyte accumulation were studied to decipher the interactions between Cd and zinc (Zn) by applying two Cd and two Zn levels (alone as well as combined). Cd²⁺ and Ca²⁺ fluxes were also measured by employing sole Cd₁₀₀ (100 μmol L⁻¹) and Zn₅₀ (50 μmol L⁻¹), and their combination with microelectrode ion flux estimation (MIFE) technique. Cd toxicity substantially reduced chlorophyll contents and maximal photochemical efficiency (Fᵥ/Fₘ) compared to control plants. Zn supplementation reverted the Cd-induced toxicity by augmenting osmoprotectants and interfering with ROS homeostasis under combined treatments, particularly in Yinni-801 genotype. Fluorescence microscopy indicated a unique pattern of live and dead root cells, depicting more damage with Cd₁₀, Cd₁₅ and Cd₁₅+Zn₅₀. Our results confer that Cd²⁺ impairs the uptake of Ca²⁺ whereas, Zn not only competes with Cd²⁺ but also Ca²⁺, thereby modifying ion homeostasis in rice plants. This study suggests that exogenous application of Zn is beneficial for rice plants in ameliorating Cd toxicity in a genotype and dose dependent manner by minimizing ROS generation and suppressing collective oxidative damage. The observations confer that Yinni-801 performed better than Heizhan-43 genotype mainly under combined Zn treatments with low-Cd, presenting Zn fortification as a solution to increase rice production.

The present study is the first attempt to evaluate the pilot and batch scale adsorption potential of siltstone (SS) and its nanocomposites with biochar (EDB/SS), magnetite nanoparticles (MNPs/SS) and MNPs/EDB/SS for Cd removal from contaminated water. The SS, EDB/SS, MNPs/SS and MNPs/EDB/SS were characterized with FTIR, XRD, BET, SEM, TEM, TGA and point of zero charge (PZC). The effects of adsorbent dosage, contact time, initial Cd concentration, pH and presence of competing ions were evaluated on the Cd removal and its adsorption. The order for Cd removal was: MNPs/EDB/SS > MNPs/SS > EDB/SS > SS (95.86–99.72% > 93.10–98.5% > 89.66.98–98.40% > 74.90–90%). Column scale experiments yielded maximum retention (95%) of Cd even after 2 h of injection at 100 mg Cd/L. The exhausted SS, EDB/SS, MNPs/SS and MNPs/EDB/SS were reused without losing significant adsorption potential. Similarly, maximum Cd adsorption (117.38 mg/g) was obtained with MNPs/EDB/SS at dose 1.0 g/L. The results revealed that coexisting cations reduced the Cd removal due to competition with Cd ions. The experimental results were better explained with Freundlich isotherm model and pseudo 2nd order kinetic models. The results revealed that SS and its composites can be used efficiently for the removal of Cd from contaminated water.

Cadmium cations (Cd²⁺) are extremely toxic to organisms, which limits the remediation of Cd by microorganisms. This study investigated the feasibility of applying biochar to protect bacteria from extreme Cd²⁺ stress (1000 mg/L). An alkaline biochar (RB) and a slightly acidic biochar (SB) were selected. SB revealed a higher Cd²⁺ removal than RB (15.5% vs. 4.8%) due to its high surface area. Addition of Enterobacter sp. induced formation of Cd phosphate and carbonate on both SB and RB surface. However, Cd²⁺ removal by RB enhanced more evidently than SB (78.9% vs. 30.2%) due to the substantial microbial regulation and surficial alkalinity. Thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and geochemical modeling (GWB) all confirmed that the formation of stable Cd phosphate on RB was superior to that in SB. These biomineralization, together with biochar pore structure, protect bacterial cells from Cd stress. Moreover, the alkalinity of biochar promoted the formation of carbonate, which strengthened the decline of Cd²⁺ toxicity. The protection by RB was also confirmed by the intense microbial respiration and biomass (PLFA). Furthermore, this protection induced a positive feedback between P-abundant biochar and Enterobacter sp.: biochar provides P source (the most common limiting nutrient) to support microbial growth; bacteria secrete more organic acids to drive P release. This study therefore elucidated the protection of bacteria by P-enriched biochar based on both physic-chemical and microbial insights.

Atmospheric wet deposition of base cations (BCs) plays a significant role in providing plant nutrients and buffering acidification. However, the temporal dynamic of wet BC deposition in China during the past two decades remain unclear. Here, we used long-term monitoring and literature data since 2000 to assess the temporal dynamics (seasonal and inter-annual variation), spatial patterns, main influencing factors, source apportionment, and capacity to neutralize the acidity of wet BC depositions at site, regional, and national scales. The results showed that total wet deposition of BCs was, on average, 2.12 keq ha⁻¹ yr⁻¹, where Ca²⁺ accounted for 65.57% of the total deposition, followed by Na⁺ (13.21%), Mg²⁺ (13.68%), and K⁺ (7.55%). Qinghai-Tibet had significantly lower BC deposition fluxes than northern, southern, and central China, as well as Inner Mongolia. Exchangeable BCs in soil, PM₁₀ in the atmosphere, energy consumption, and cement production are significantly related to wet BC deposition, which account for 79.17% of the variation in the spatial deposition of BC. Influenced by the strategies to control acid rain and particulate matter in China, interannual variations showed a stabilization trend after a continuous decline from 2000 to 2017, which can be explained by inter-annual changes in PM₁₀, energy consumption and cement production. Statistical methods confirmed that 45.95% of wet BC deposition was derived from crustal contributions, 27.78% from sea salt sources, and 26.27% from anthropogenic sources. Furthermore, we found that wet deposition of BCs neutralized 84.85% of the acidity due to NO₃⁻ and SO₄²⁻ depositions. Under the emissions reduction strategy, there has been a decrease in the deposition of BC. However, SO₄²⁻ and NO₃⁻ depositions decreased faster than BC deposition, which buffered a higher proportion of acidic depositions. Our findings contribute to an improved understanding of wet BC deposition in China, an evaluation of their capacity to neutralize acidity, and important parameters for acidification models.

The accumulation of cadmium (Cd) in rice grains is closely associated with the content of mineral nutrients and amino acid metabolism, but the causal link among them is unclear. Profiles of amino acids (AAs) and quantities of essential nutrients in grains from early and late rice cultivars grown at four sites with different Cd levels were analyzed in the present study. Hazard quotients (HQs) for consumers by intake of rice from late cultivars were much higher than that from early cultivars at sites with soil Cd content of 0.25, 0.61 and 0.84 mg kg⁻¹. Cadmium accumulation in grains resulted in a sharp reduction of total essential AAs and non-essential AAs in both early and late rice cultivars. High-Cd-accumulating (HCA) cultivars had significantly higher level of glutamate (Glu) than low-Cd-accumulating (LCA) cultivars when rice Cd content was less than 0.20 mg kg⁻¹. However, Glu level in grains dramatically declined with the accumulation of Cd, which subsequently leaded to the reduction of other AAs. Cadmium content was well predicted by five amino acids (i.e., Glu, Alanine, Phenylalanine, Glycine and Threonine) or four essential elements (Ca, Fe, Mn and Zn) when rice Cd was less than 0.80 mg kg⁻¹. Amino acids played more important roles than nutrients in Cd accumulation. When Cd content was in the range of 0.40–1.16 mg kg⁻¹, the Mn content in rice increased significantly with the increase of Cd content, while the Glu content dropped down synchronously. Remarkably, the ratio between Mn and Glu displayed the highest direct path coefficient on Cd accumulation than any single cation or amino acid. These results indicate that high capacity in synthesizing Glu and concentrating Mn is the determinant factor for Cd accumulation in rice grains, and abundant Glu in aleurone layer may alleviate Cd toxicity by forming Glu-Cd complex.

The release of fine particles from biochar materials applied in the environment may have important environmental implications, such as mobilization of environmental contaminants. In natural environments biochar fine particles can undergo various transformation processes, which may change their surface chemistry and consequently, the mobility of the particles. Here, we show that sulfide reduction can significantly alter the transport of wheat-straw- and pine-wood-derived biochar fine particles in saturated porous media. Counterintuitively, the sulfide-reduced biochar particles exhibited greater mobility in artificial groundwater than their non-reduced counterparts, even though reduction led to decrease of surface charge negativity and increase of hydrophobicity (from the removal of surface O-functional groups), both should favor particle deposition, as predicted based on extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory. Using transport experiments conducted in single-cation background solutions containing K⁺, Mg²⁺ or Ca²⁺ under different pH conditions, we show that the surprisingly greater mobility of sulfide-reduced biochar particles was attributable to the removal of surface carboxyl groups during reduction, as this markedly alleviated particle deposition through cation bridging, wherein Ca²⁺ acted as the bridging agent in linking the surface O-functional groups of biochar particles and quartz sand. These findings show the critical roles of surface properties in dictating the mobility of biochar fine particles and call for further understanding of their transport properties, which apparently cannot be simply extrapolated based on the findings of other (engineered) carbonaceous nanomaterials.

Sawdust wastes were used as precursors to prepare adsorbents by combustion and pyrolysis for experimental and mechanism studies and determine the potential of biomass extracted from agro-industrial residues for Pb-polluted soil remediation. Pot experiments were conducted on contaminated soils near Pb–Zn mining with sawdust ash (SA) and sawdust biochar (SB) in different proportions and dosage ratios. Studies have indicated that the application of biomass materials can enhance the adsorption, complexation and precipitation of Pb cations in soil and reduce the mobility of Pb. The concentrations of SPLP-Pb and DTPA-extractable Pb in amended soils were the lowest under 1% 1:2 and 5% 1:1 treatment, respectively. Results of fraction extraction and XANES analysis showed that the materials change the main forms of Pb in soil. Moreover, the binding behavior of Pb with organic matter increases the proportion of Pb (Ac)₂, leading to the transformation of high toxicity Pb-compounds into precipitates and complexes. The remediation methods of 2% 1:2 and 5% 1:2 were better than those of other methods in stabilizing Pb in soil. This study indicated that heat-treated sawdust can be used for Pb-polluted soil remediation, which is a type of environmental remediation measure with considerable ecological potential.

Shortly after an atmospheric release, the interception of radionuclides by crop canopies represents the main uptake pathway leading to food chain contamination. The food chain models currently used in European emergency decision support systems require a large number of input parameters, which inevitably leads to high model complexity. In this study, we have established a new relationship for wet deposited radionuclides to simplify the current modelling approaches. This relationship is based on the hypothesis that the stage of plant development is the key factor governing the interception of radionuclides by crops having horizontally oriented leaves (planophile crops). The interception fraction (f) and the leaf area index normalized (fLAI) and mass normalized (fB) interception fractions were assessed for spinach (Spinacia oleracea) and radish (Raphanus sativus) at different stages of plant development and for different contamination treatments and plant densities. A database of 191 f values for Cs-137 and Th-229 was built and complemented with existing literature covering various radionuclides and crops with similar canopy structure. The overall f increased with the plant growth, while the reverse was observed for fB. The fLAI significantly decreased by doubling the contaminated rainfall deposited. Fitting a multiple linear regression to predict the f value as a function of the standing biomass (B), and the radionuclide form (anion and cation) led to a better estimation of the interception (R² = 81%) than the ECOSYS-87 model (R² = 35%). Hence, the simplified modelling approach here proposed seems to be a suitable risk assessment tool as fewer parameters will minimize the model complexity and facilitate the decision-making procedures in case of emergencies, when countermeasures need to be identified and implemented promptly.