The Phytotoxicity Of Added Copper And Nickel To Soils And Predictive Models

The phytotoxicity of copper and nickel added to Chinese soils with a wide range of properties was studied in order to find the main soil factors affecting their phytotoxicity in soils, and to develop the predictive models for the phytotoxicity using soil main properties. The mechanistic model-terrestrial biotic ligand model (TBLM) was also developed in order to supply a useful method for risk assessment of metals in soils.First, the eight different plant species were selected for test to obtain species sensitivity distribution for copper (Cu) and nickel (Ni) toxicity in soils, it was found that bok choy and mustard are the most sensitive species; barley and tomato are medium sensitive species for Cu and Ni toxicity among these eight plants. The species sensitivity distribution is strongly affected by soil properties and different Cu and Ni elements.Three plant species, bok choy, barley and tomato, were chosen for studying the influence of soil properties on Cu and Ni toxicity in 17 Chinese soils under controlled environmental conditions with or without leaching treatments. The effective concentration of added Cu and Ni causing 50% inhibition (EC50) for plant growth ranged from 10 to 2,519 mg/kg in unleached soils and from 18 to >2,381 mg/kg in leached soils, respectively, indicating significant effects of soil factors on Cu and Ni phytotoxicity. Empirical regression models developed using soil factors and phytotoxicity thresholds showed that soil pH is the best predictor for Cu and Ni toxicity, explaining 26%~37% of variances for Cu phytotoxicity and 39%~78% of variances for Ni phytotoxicity. The organic carbon contents (OC) and effective cation exchange capacity (eCEC) are the second important predictors and could improve model predictivity further by explaining 10~55% of variances in EC50 values. The mutual validation between our and European regression models showed that our models can predict the phytotoxicity thresholds in European soils well; however, European models could not predict the phytotoxicity thresholds in Chinese soils. These quantitative relationships between Cu and Ni toxicity and soil properties are quite helpful to develop soil-specific guidance on Cu and Ni toxicity thresholds.Leaching can decrease Cu and Ni toxicity in soils through increasing soil solution pH, decreasing ionic strength in soil solutions. However, the effect of leaching on decreasing Ni toxicity was much bigger than that on Cu. Leaching increased EC50 values by an average factor ~1.2 for Cu and >1.3 for Ni. Especially for soils with pH >8.2, leaching dramatically decreased Ni phytotoxicity.The free Cu ion activity measured using cupric ion selective electrode (ISE) based on 34 soil samples and free Ni using Donna membrane technique (DMT) based on 6 samples were compared with results predicted using vMINTEQ incorporating NICA-Donnan model and WHAM 6 based on soil solution data, results showed that the best consistence produced between measured and predicted free Cu activity when using 30% DOM as active fluvic acid (AFA) with default parameters for NICA-Donnan model, or 65% DOM as AFA with optimized stability constant logKCuFA 1.5 for WHAM 6. The Measured free Ni agreed with predicted free Ni when 30% DOM as AFA and default constants were used in WHAM 6 and NICA-Donnan model.In solution culture experiment of Ni toxicity to barley root elongation, it was found that NiHCO3+ is another toxic species besides Ni2+ ion, especially at high-pH conditions. The DPS statistic software was used to fit binding constants between different ions and acceptor sites in barley root surface based on biotic ligand model (BLM) theory framework, the best fit results for binding constants were obtained as follows: KCaBL = 1.60, KMgBL = 4.01, KHBL = 4.29, KNiBL = 4.83, KNiHCO3BL = 5.36.The terrestrial biotic ligand model (TBLM) for Cu and Ni derived from 17 of leached soil samples was developed using above binding constants and corresponding soil solution parameters (Ca, Mg, H, DOC, Fe, Al, Cl- and SO42-), and results showed that predicted and measured barley root elongation (%) can match quite well with a value of r2≥0.90. The advantage of TBLM in the present study beyond European TBLM is the introduction of a series of high-pH soils samples, which will help a lot on formulating guidance of soil environment risk assessments not only for China but for the other areas around world.