Comparative effects of Tagetes patula L. extraction, mercapto-palygorskite immobilisation, and the combination thereof on Cd accumulation by wheat in Cd-contaminated soil

A field trial for remediation of multi-metal contaminated soils using the combination of fly ash stabilization and Zanthoxylumbungeanum- Lolium perenne intercropping system

Insitu stabilization and phytoextraction are considered as two convenient and effective technologies for the remediation of toxic elements (TEs) in soils. However, the effectiveness of these two remediation technologies together on the bioavailability and phytoextraction of TEs in field trials has not been explored yet. Specifically, the remediation potential of fly ash (FA; as stabilizing agent) and ryegrass (as a TE accumulator) intercropped with a target crop for soil polluted with multiple TEs has not been investigated yet, particularly in long-term field trials. Therefore, in this study, a six-month combined remediation field experiment of FA stabilization and/or ryegrass intercropping (IR) was carried out on the farmland soils contaminated with As, Cd, Cr, Cu, Hg, Ni, Pb and Zn where Zanthoxylumbungeanum (ZB) trees as native crops were grown for years. The treatments include soil cultivated alone with ZB untreated- (control) and treated-with FA (FA), produced by burning lignite in Shaanxi Datong power plant, China, soil cultivated with ZB and ryegrass untreated- (IR) and treated-with FA (FA + IR). This was underpinned by a large-scale survey in Daiziying (China), which showed that the topsoils were polluted by Cd, Cu, Hg and Pb, and that Hg and Pb contents in the Zanthoxylumbungeanum fruits exceeded their allowable limits. The TEs contents in the studied FA were lower than their total element contents in the soil. The DTPA-extractable TEs contents of the remediation modes were as follows: FA < FA + IR < IR < control. Notably, TEs contents in the ZB fruits were lowest under the FA + IR treatment, which were decreased by 27.6% for As, 42.3% for Cd, 16.7% for Cr, 30.5% for Cu, 23.1% for Hg, 15.5% for Ni, 33.2% for Pb and 38.1% for Zn compared with the control treatment. Whereas the FA + IR treatment enhanced TEs contents in ryegrass shoots and roots, and the TEs contents in ryegrass shoots were below their regulatory limits for fodder crops. The findings confirmed that the combined remediation strategy, i.e., FA (with low content of TEs) stabilization effect and intercropping of ZB (target crop) and ryegrass (accumulating plant) could provide a prospective approach to produce target plants within safe TEs thresholds with greater economic benefits, while remediating soils polluted with multiple TEs and mitigating the potential ecological and human health risk. Those results are of great applicable concern.

Soil addition of MnSO4 reduces wheat Cd accumulation by simultaneously increasing labile Mn and decreasing labile Cd concentrations in calcareous soil: A two-year pot study

Cadmium (Cd) pollution of wheat fields is a serious environmental and health problem that warrants attention. Manganese (Mn)-containing materials are considered effective for inhibiting Cd accumulation in Cd-contaminated acidic soils. However, information on the long-term remediation effects of Mn fertilizers on Cd accumulation in wheat and on the microbial community in calcareous soils remain limited. Here, a two-year pot experiment was conducted to assess the performance of 0.05-0.2% MnSO₄ addition in Cd-contaminated calcareous soils (total Cd concentration: 3.65 mg/kg) on Cd accumulation in wheat as well as on the soil bacterial community. The formation of Mn oxides and transformation of exchangeable Cd to stable Cd fractions confirmed that the application of MnSO₄ significantly decreased CaCl₂-extractable Cd concentrations in soil (0-47.08%). In addition, MnSO₄ addition improved the antagonistic effect of Cd and Mn ions in the wheat rhizosphere by increasing the available Mn concentration in the soil (1.04-3.52 times), thereby significantly reducing wheat Cd accumulation by 24.66-54.70%. Notably, the addition of MnSO₄ did not affect the richness and diversity (P > 0.05) but altered the composition and function of bacterial communities, especially those involved in metabolism and genetic information processing. Importantly, the effects of MnSO₄ on Cd immobilization in soil (10.66-47.08%) and the inhibition of Cd accumulation in wheat (12.13-54.30%) can last for two years after one addition. Furthermore, the maximum decrease in Cd concentration in grains was found in the low-Cd wheat cultivar, with values of 31.39-54.70% and 19.94-54.30% in the first and second years, respectively. Based on the present findings, the combination of MnSO₄ with a low-Cd wheat cultivar is effective for the safe utilization of Cd-contaminated calcareous soils.

Removal mechanisms of Cd from water and soil using Fe-Mn oxides modified biochar

The development of remediation materials simultaneously suitable for Cd-contaminated water and soil is of great significance. In this study, the functional biochar (FM-RBC and FM-DBC) was prepared using branch and durian shell biochar (RBC and DBC, respectively) with iron-manganese oxide (Fe-Mn oxide) modification. The behaviors and mechanisms of Cd adsorption and stabilization in water and alkaline soil treated with FM-RBC and FM-DBC were explored. The results showed that the adsorption capacities of RBC and DBC for Cd had increased by 40-80 mg/g after the Fe-Mn oxide modification. The Cd adsorption was conformed to pseudo-second-order kinetic and the Langmuir isothermal models. After 35 days of soil cultivation, the maximum reduction rate of DTPA-Cd occurred in 3% FM-DBC treatments (37.73%), followed by in 3% FM-RBC (30.08%), all of which were significantly higher than that observed in 3% BC treatments (12.55-18.91%). Notably, the FM-RBC and FM-DBC treatments promoted the conversion of the exchangeable to the carbonate-bound and Fe/Mn oxyhydroxide fractions of Cd. The XRD, FTIR, and XPS analyses demonstrated that the loading amount of Fe-Mn oxide was positively correlated with the oxygen-containing functional group of biochar. CdO, Cd₂Mn₃O₈ and CdCO₃ were loaded on the FM-BC, indicating the existence of two main adsorption mechanisms: (1) the complexation with M-O (M: Fe, Mn) and acid oxygen-containing functional groups, (2) the precipitation with carbonate of Cd. In this work, we prepared two functional biochar that rapidly removes Cd from water and effectively fixes Cd in alkaline soil, thus, debasing the risk of Cd entry into the food chain.

Soil application of manganese sulfate effectively reduces Cd bioavailability in Cd-contaminated soil and Cd translocation and accumulation in wheat

Cadmium (Cd) pollution in wheat fields has caused serious food safety issues in China. Manganese (Mn)-containing materials have been widely used in paddy fields to reduce Cd accumulation in rice. However, the remediation effects of MnSO₄ in wheat fields have not been well studied and the underlying mechanisms are poorly understood. Our field experiment showed that the application of 0.1% and 0.2% MnSO₄ in soil reduced Cd concentrations significantly in wheat root, stem, leaf, and grain by 26.67-30.76%, 15.78-29.30%, 22.03-30.66%, and 30.57-50.55%, respectively, while increasing Mn concentrations significantly in these wheat tissues. MnSO₄ application significantly increased soil available Mn content by 3.78-6.19 times, the free Mn oxides and amorphous Mn oxides by 1.72-10.38 times, and Mn oxides bound Cd contents by 10.23-39.55%, resulting in a reduction of Cd availability by 30.11-40.78%. Simultaneously, MnSO₄ treatment altered the chemical forms of Cd and Mn, promoted the soluble protein concentration, and decreased the malondialdehyde (MDA) content in wheat roots. Additionally, soil application of MnSO₄ down-regulated the expression of TaNramp5, TaHMA2, and TaLCT1 in wheat roots, mediating the reduction of wheat root Cd concentration, and increased the sequestration of Cd into vacuoles by up-regulating the expression of TaHMA3. These findings add to the current knowledge of how MnSO₄ affects Cd mobilization and absorption via different mechanisms occurring both in the soil medium and at the plant level. This research indicates that soil application of MnSO₄ has great potential to remediate Cd-contaminated wheat fields.

Metal(loid)s Spatial Distribution, Accumulation, and Potential Health Risk Assessment in Soil-Wheat Systems near a Pb/Zn Smelter in Henan Province, Central China

To understand the influence of Pb/Zn smelter on surrounding environment, 110 soil and 62 wheat grain samples (62 paired samples) were collected nearby a Pb/Zn smelter in Jiaozuo City, Henan Province, China. The content and spatial distribution of metal(loid)s in the soil-wheat system, and the potential health risk via consumption of wheat grains were determined. Results showed that the average content of Pb, Cd, As, Cu, Zn, and Ni in soil were 129.16, 4.28, 17.95, 20.43, 79.36, and 9.42 mg/kg, respectively. The content of Cd in almost all soil samples (99.1%) exceeded the national limitation of China (0.6 mg/kg). Spatial distribution analysis indicated that atmospheric deposition might be the main pollution source of Pb, Cd, As, and Zn in soil. In addition, the average content of Pb, Cd, As, Cu, Zn, and Ni in wheat grain were 0.62, 0.35, 0.10, 3.7, 35.77, and 0.15 mg/kg, respectively, with the average Pb and Cd content exceeding the national limitation of China. The average bioaccumulation factor of these metal(loid)s followed the following order: Zn (0.507) > Cu (0.239) > Cd (0.134) > Ni (0.024) > Pb (0.007) > As (0.006). Health risk assessment indicated that the average noncarcinogenic risk of children (6.78) was much higher than that of adults (2.83), and the carcinogenic risk of almost all wheat grain is higher than the acceptable range, with an average value of 2.43 × 10−2. These results indicated that humans who regularly consume these wheat grains might have a serious risk of noncarcinogenic and carcinogenic diseases.