Assessing Heavy Metal Bioavailability In Contaminated Soils Using Diffusive Gradients In Thin Films (DGT) Technique With Different Binding Phases

Heavy metal pollution in soils poses a serious threat to the quality and safety of agriculturalproducts. Thus developing effective methods for evaluate heavy metal bioavailability has been a hottopic in the field of agricultural environment research. The extraction of heavy metals depended onelement types, soil texture, pH, temperature, the nature of the solvent, and so on. There was still a lackof effective uniform standards of extraction bioavailable heavy metals from soil. A new technique,diffusive gradients in thin films (DGT), measures labile species and bioavailability of heavy metals insitu. Many studies have shown the heavy metals measured by DGT were closer to the bioavailabilityconcentrations, and DGT technique could better predict the potential runoff of heavy metals in plantsthan conventional methods. The intellectual property right of DGT technique was belongs to U.K., andthe masteries were generally product by U.K. and U.S.A., which needed to be imported and resulting inhigher cost, so it was hard to promote it in a large area in China. Developing DGT technique with ourown intellectual property right, producing DGT device localization, and promoting the application ofDGT technique in China in order to guide agricultural production, is one of the problems to be solved.The diffusive phase and binding phase of DGT device were absorbed and improved based on theDGT technique introduced form foreign country. Nine kinds of commercially available membraneswere selected as diffusive layers to study the diffusion coefficients of Cd in these diffusive layers andthe impact of ionic strength on the diffusion coefficients of Cd, with sodium polyacrylate (PAAS) asbinding phase.16natural Cd contaminated soils with diverse properties were collected, and ryegrasswas grown on these soils by pot experiment. Prediction models for evaluating cadmium accumulation inryegrass were built based on Cd bioavailable concentrations measured by DGT technique with twodifferent binding phases, which was also compared with prediction models built by traditional chemicalmethods. The bioavaliability of lead in the rhizosphere soils of Moraceae was accumulated andmeasured by different binding phases DGT devices. Multivariate analyses were performed andregression models were established, which also consider the impact of the physical and chemicalindicators of soils. The bioavailability concentrations of Cd in rhizosphere soils of ryegrass andpokeweed grown under fungal inoculation and elevated CO₂were measured by DGT withtwo different binding phases.The main conclusions were as follows:(1)The dialysis membranes with14000,7000,5000,3500and1000interception molecularweight, fluoride membrane, polyethersulfone membrane, cellulose acetate membrane and mixedcellulose membrane were chosen as diffusive phases. The diffusion coefficients of Cd in these9kindsof diffusive phases were determined with sodium polyacrylate (PAAS) as binding phase. In general,the diffusion coefficients had a flux under low ionic strength (ionic strength greater than5mM), andthe diffusion coefficients had no change nearly when the ionic strength higher than25mM. Considerfrom all aspects, such as cost, the diffusive flux, scientific and so on, it was the best choose to usedialysis membranes with14000interception molecular weight as diffusive layer in liquid binding phaseDGT technique. (2)Multiple regression prediction models for evaluating cadmium accumulation in ryegrasswere built based on Cd bioavailable concentrations extracted by DGT technique and traditional methods,which also the impact of pH, cation exchanged capacity (CEC), organic materials (OM) and textureof the soils. Results showed that prediction models based on chelex100-DGT and PAAS-DGTtechniques were significantly better than those built by traditional techniques (soil solution, CaCl2,EDTANa2and HAc extractions). Results showed that pH and CEC had significant effects on predictionmodels built by4traditional methods, but there was little effect on the prediction model based on DGTtechnique. DGT measurement incorporated the main factors affected bioavailability, and could betterpredict the potential runoff of Cd from soil and the accumulation of Cd in ryegrass than traditionalchemical methods. Results also showed that DGT technique with dialysis membranes with14000interception molecular weight as diffusive layer and PAAS as binding phase could predict the potentialrunoff of Cd from soil and the accumulation of Cd in s ryegrass, which expanded the applicable range ofDGT technique.(3)The bioavaliability of lead in the rhizosphere soils of Moraceae was accumulated and measuredby different binding phases DGT devices (chelex100-DGT and PAAS-DGT). Multivariate analysisshowed that the regression models established by the two binding phase DGT were reliable, and theadjusted coefficients were0.87,0.89,0.96and0.95, respectively. Predicted results from these modelsincorporated the main factors affecting available Pb in soils, such as pH, cation exchanged capacity(CEC), soil organic matter (OM) and texture. Results showed that both DGT devices could predict thepotential runoff of Pb from soil and the accumulation of Pb in leaves of Moraceae, which expanded theapplicable range of DGT technique.(4)Elevated CO₂and fungal inoculation, either singly or in combination, could significantlyincreased plant biomass and total Cd uptake per pot ryegrass and pokeweed. The bioavailable Cdconcentrations in the rhizosphere soils measured by DGT were increased, which revealed the directcause that elevated CO₂and fungal inoculation could enhance the heavy metals accumulated in plants.Our present study showed that the shoot and root dry weight biomass of ryegrass grown at elevated CO₂increased by42.08–89.63and17.93–51.43%, respectively, compared to the ambient CO₂control. Bycontrast, the increase in DGT-measured Cd concentrations in the rhizosphere soil of ryegrass at elevatedCO₂was only2.01–25.17and15.79–38.31%, respectively, compared to the ambient CO₂control. It isclear that there was a substantial difference between the increased rate of the shoot and root dry weightbiomass of ryegrass grown at elevated CO₂, and the increased magnitude of DGT-measured Cdconcentrations in the rhizosphere soil of ryegrass elevated CO₂. This implies that the apparent increasein size of the bioavailable metal pools in the rhizosphere of ryegrass due to elevated CO₂might not meetthe potential for plant uptake of metals in association with the plant biomass increase and, as a result, a'dilution effect' took place under elevated CO₂. The situation was similar for pokeweed.In conclusion, chelex100-DGT and PAAS-DGT techniques could be used as an effective andaccurate method to characterize and predict heavy metal bioavailable concentrations in soils.PAAS-DGT technique with14000interception molecular weight dialysis membranes as diffusive layer could predict the potential runoff of Cd and Pb from soil and the accumulation of Cd and Pb in plants,which expanded the applicable range of DGT technique, and provided theoretical basis and technicalsupport for promoting the application of DGT technique in China.