Evaluation of Different Types and Amounts of Amendments on Soil Cd Immobilization and its Uptake to Wheat

Mitigation of the mobilization and accumulation of toxic metal(loid)s in ryegrass using sodium sulfide

Remediation of soils contaminated with toxic metal(loid)s (TMs) and mitigation of the associated ecological and human health risks are of great concern. Sodium sulfide (Na2S) can be used as an amendment for the immobilization of TMs in contaminated soils; however, the effects of Na2S on the leachability, bioavailability, and uptake of TMs in highly-contaminated soils under field conditions have not been investigated yet. This is the first field-scale research study investigating the effect of Na2S application on soils with Hg, Pb and Cu contents 70-to-7000-fold higher than background values and also polluted with As, Cd, Ni, and Zn. An ex situ remediation project including soil replacement, immobilization with Na2S, and safe landfilling was conducted at Daiziying and Anle (China) with soils contaminated with As, Cd, Cu, Hg, Ni, Pb and Zn. Notably, Na2S application significantly lowered the sulfuric-nitric acid leachable TMs below the limits defined by Chinese regulations. There was also a significant reduction in the DTPA-extractable TMs in the two studied sites up to 85.9 % for Hg, 71.4 % for Cu, 71.9 % for Pb, 48.1 % for Cd, 37.1 % for Zn, 34.3 % for Ni, and 15.7 % for As compared to the untreated controls. Moreover, Na2S treatment decreased the shoot TM contents in the last harvest to levels lower than the TM regulation limits concerning fodder crops, and decreased the TM root-to-shoot translocation, compared to the untreated control sites. We conclude that Na2S has great potential to remediate soils heavily tainted with TMs and mitigate the associated ecological and human health risks.

Effects of application of rice husk biochar and limestone on cadmium accumulation in wheat under glasshouse and field conditions

Cadmium (Cd) has seriously threatened the safe production of food crops. Passivator amendments are commonly used to control the soil Cd availability. Yet, few studies are tested to explore the effect of the combination of various amendments. Here, we investigated the effects of different amendments (2% rice husk biochar, 2% limestone, and 1% rice husk biochar + 1% limestone) on the growth and Cd accumulation of wheat in pot and field experiments. The results showed that under the low soil Cd condition, the maximum increase of soil pH (1.83) was found in the limestone treatment compared to CK in pot experiment. Compared with the CK, the treatment of rice husk biochar decreased soil Cd availability and grain Cd content by about 25% and 31.2%, respectively. In contrast, under high soil Cd condition, the highest soil pH was observed in limestone, while the lowest soil Cd availability and grain Cd concentrations were found in rice husk biochar treatment. In the field experiment, the treatment of 1% rice husk biochar + 1% limestone caused a significant increase of soil pH by about 28.2%, whereas the treatment of 2% rice husk biochar reduced soil Cd availability and grain Cd content by about 38.9% and 38.5% compared to the CK. Therefore, rice husk biochar showed great potential to reduce Cd availability and ensure safe food production.

Humates mitigate Cd uptake in the absence of NaCl salinity, but combined application of humates and NaCl enhances Cd mobility & phyto-accumulation

Cadmium is among the critical pollutants easily taken up from contaminated media by plants, which can be exploited in the phytoremediation of Cd-contaminated resources, but is also an obstacle in producing food with low Cd content. Crucial variables governing Cd biogeochemistry are complex humates (HA) and chlorides, but the underlying interactions are poorly understood. The aim was to determine the impacts of HA (0-60 mg/L) and NaCl (0-30 mM) on Cd biochemistry in contaminated (2.0 μM Cd) rhizosphere solution and Cd accumulation in various tissues of strawberry (Fragaria x ananassa). The results show that salinity (vs. non-saline NaCl₀ control) suppressed vegetative and yield parameters, but increased dry matter and Na, Cl and Cd concentration/accumulation in most of the analysed tissues. The HA application in the NaCl₀ treatment decreased tissue Cd content; however, at the highest application rates of NaCl and HA, there were increases in the tissue Cd concentration (by 70 %, 100 % and 120 % in crowns, leaves and fruits, respectively) and accumulation (by 110 %, 126 % and 148 % in roots, fruits and leaves, respectively) in comparison to the control (NaCl₀HA₀). Tissue Cd concentration/accumulation decreased in the order: roots>crowns>leaves>fruits; the same accumulation pattern was noted for Na and Cl, suggesting that Cd-Cl complexes may represent a major form of Cd taken up. Chemical speciation calculations revealed that the proportions of various Cd forms varied multi-fold across the treatments; in the control (without NaCl and HA), Cd²⁺ dominated (86 %), followed by CdHPO₄ (6.5 %), CdSO₄ (6.2 %) and CdNO₃⁺. In other treatments the proportion of Cd²⁺ decreased with a corresponding increase of Cd-Cl (from 0.02 % in control to 57 % in Cd + NaCl₃₀ treatment) and Cd-HA (from 0 % in control to 44 % in Cd + HA₆₀ treatment), which was associated with higher Cd phytoaccumulation. The results represent a theoretical basis for phytoremediation studies and for producing low-Cd food in relatively complex matrices (contaminated soils, reused effluents); in the absence of salinity, amelioration with humates has a great potential to mitigate Cd contamination.

Effects of exogenous additives on wheat Cd accumulation, soil Cd availability and physicochemical properties in Cd-contaminated agricultural soils: A meta-analysis

Cadmium (Cd) contamination in wheat is a serious issue. The application of exogenous additives can effectively inhibit Cd bioavailability in soil and decrease Cd accumulation in wheat. However, a comprehensive and quantitative analysis of how additives affect wheat Cd accumulation, wheat yield, soil Cd availability, and soil properties is lacking. We conducted a meta-analysis of 65 peer-reviewed papers published before April 2021 to investigate how additives application affects Cd accumulation in wheat and soil Cd availability. The results indicated that most additives application decreased the diethylenetriaminepentaacetic acid extractable-Cd content (5.27-56.33%) in the soil, and wheat grain and root Cd concentrations (0.03-129.87% and 0.42-52.84%, respectively); the pH values of wheat-grown soil and the properties of the additives affected the reduction percentage. Overall, most wheat-grown soils were calcareous soil (42 peer-reviewed papers); in calcareous soil, the magnitude of the Cd reduction in wheat grain was the highest under treatments with clay minerals (129.87%) due to clay modification, followed by composite (75.36%) and phosphorus materials (73.55%). Moreover, most additives application increased wheat grain yield by 0.03-51.84%, which was attributed to the positive effects of additives on wheat antioxidant capacity, photosynthesis, respiration, and nutrient uptake. Additives application increased the pH value of acidic wheat soil, and positively affected the electrical conductivity, cation exchange capacity, and organic carbon content of the wheat grown soil. In addition, regression analysis showed that soil available Cd was negatively correlated with the pH value with additives application in acidic soil (r² = 0.43), while a non-significant correlation was observed in neutral and calcareous wheat soils (r² = 0.017 and 0.016, respectively). The results of this study can assist in the selection, modification, and utilisation of additives to remediate Cd-contaminated wheat soils.

Assessing the Capability of Chemical Ameliorants to Reduce the Bioavailability of Heavy Metals in Bulk Fly Ash Contaminated Soil

In-situ rehabilitation of fly ash at dumping sites has rarely been addressed for crop production due to growth-related constraints, largely of heavy metal (HM) contamination in soils and crops. Current communication deals with a novel approach to identify a suitable management option for rejuvenating the contaminated soils. In this background, a 60-days incubation experiment was conducted with different fly ash-soil mixtures (50 + 50%, A1; 75 + 25%, A2; 100 + 0%, A3) along with four ameliorants, namely, lime (T1), sodium sulphide (T2), di-ammonium phosphate (T3), and humic acid (T4) at 30 ± 2 °C to assess the ability of different fly ash-soil-ameliorant mixtures in reducing bio-availability of HMs. Diethylenetriaminepentaacetic acid (DTPA)-extractable bio-available HM contents for lead (Pb), cadmium (Cd), nickel (Ni), and chromium (Cr) and their respective ratios to total HM contents under the influence of different treatments were estimated at 0, 15, 30, 45, and 60 days of incubation. Further, the eco-toxicological impact of different treatments on soil microbial properties was studied after 60 days of experimentation. A1T1 significantly recorded the lowest bio-availability of HMs (~49-233% lower) followed by A2T1 (~35-133%) among the treatments. The principal component analysis also confirmed the superiority of A1T1 and A2T1 in this regard. Further, A1T1 achieved low contamination factor and ecological risk with substantial microbial biomass carbon load and dehydrogenase activity. Thus, liming to fly ash-soil mixture at 50:50 may be considered as the best management option for ameliorating metal toxicity. This technology may guide thermal power plants to provide the necessary package of practices for the stakeholders to revive their contaminated lands for better environmental sustainability.

Biological effects of biochar and zeolite used for remediation of soil contaminated with toxic heavy metals

Biochar and zeolite are widely used in the remediation of soil contaminated with toxic heavy metals. However, the interaction of these amendments and their effects on grass productivity have not been comprehensively summarized. The aim of this study was to investigate the biological effects of zeolite and biochar used as soil amendments in the process of remediating soil contaminated with Cd, Pb and Zn. In a pot experiment, the following treatments were applied: zeolite, biochars produced at temperatures of 350 °C and 550 °C, mixtures of biochars and zeolite, and a control treatment without any amendments. The soil amendments were tested on two grass species: tall fescue (Festuca arundinacea Schreb.) and cocksfoot (Dactylis glomerata L.). The root morphometric parameters and aboveground production were determined in 2017 and 2018.Higher biomass production was observed in the tested grasses in the treatments with zeolite alone (0.229 kg DM m^-2) or mixed with the biochars (0.239 kg DM m^-2) than in control treatment (0.029 kg DM m^-2). Zeolite used in contaminated soil significantly affected root biomass and root morphology parameters. Zeolite application resulted in significantly higher root biomass (2.30 mg cm^-3) and root length (76.61 cm cm^-3) than those in the treatments without zeolite (0.29 mg cm^-3 and 6.90 cm cm^-3). Biochar as a soil amendment did not affect most root morphometric parameters. The application of biochars only slightly reduced the root diameter of cocksfoot. The root diameter of tall fescue was similar in all treatments (0.075 mm) except the control (0.063 mm) and biochar 550 treatments (0.067 mm), in which slightly thinner roots were observed.

Toxicological effects of single and joint sulfamethazine and cadmium stress in soil on pakchoi (Brassica chinensis L.)

The combined pollution of heavy metals and antibiotics in soil has attracted increasing attention due to their negative effects on plant growth. The aims of this study were to evaluate the phytotoxicity of single and combined sulfamethazine (SMT) and cadmium (Cd), selected as target pollutants in soil, on growth and physiological response of pakchoi (Brassica chinensis L.). Results revealed that the soil spiked with 10 mg kg^-1 Cd inhibited the pakchoi growth regardless of SMT addition. The combined effect of SMT and Cd stress on uptake of SMT or Cd by pakchoi were concerned with their combined concentration. The combined influence of high concentrations SMT and Cd (1 and 10 mg kg^-1) exposure on the Cd content of pakchoi showed antagonistic effects and synergistic effects, respectively. Besides, oxidative substances and enzyme activity of pakchoi tissue were affected by Cd and SMT exposure in the soil, particularly by their joint stress. This mainly expressed as the increase of malondialdehyde (MDA), H₂O₂ content and antioxidant enzyme activity (superoxide dismutase (SOD), peroxidase (POD), catalase (CAT)), which could be ascribed to the induction of Cd and SMT stress. Additionally, the SMT-Cd combined stress caused more reduction in nutrients (vitamin C and sugar) of pakchoi than the correspondingly single Cd stress. In conclusion, the SMT and Cd in soil lead to their accumulation and oxidative damage in pakchoi, which disturb the antioxidant defense system and ultimately adversely affect growth and quality of pakchoi.