Phosphorus-Induced Immobilization Of Lead, Copper, And Znic In A Multi-Metal Contaminated Soil: Pot And Hydroponic Experiments

Industrial and agricultural activities have released large amounts of heavy metals into soils, leading to increasingly serious issues of heavy metal contamination in soils which have been threatening our living environment and sustainable development of agriculture. Considerable researches have been made to evaluate the effectiveness of chemical immobilization techniques based on phosphorus amendments that have been widely and successfully used in immobilization of one single metal (e.g. Pb) in contaminated soils. However, soils are usually contaminated by multiple heavy metals. Therefore, application of P amendments in the immobilization of multiple metals and corresponding investigations about the effects of environmental conditions on the stability and bioavailability of the initially P-induced immobilized metals in the contaminated soils are of more significance.This study was conducted to determine effects of triple superphosphate fertilizer (TSP), phosphate rock tailing (PR), and their combination (P+T) on the mobility of Pb, Cu, and Zn in a multi-metal-contaminated soil under continuous acidic rain. Chinese cabbage (Brassica rapa subsp. chinensis) (metal-sensitive) and Chinese kale (Brassica alboglabra Bailey) (metal-resistant) were introduced to examine the effects of planting on the stability of Pb, Cu and Zn in P-amended soils and the effects of P amendments on distribution and transformation of Pb, Cu and Zn in the soil-plant system. Compounds formed on the surface of hydroponic plant roots before and after immobilization were analyzed and compared with spectroscopic and microscopic techniques (XRD, SEM/EDS) so that we could study the immobilization mechanism of heavy metals by P amendments. The main results are as follows:(1) Simulated rainwater Leaching experiment indicated all three P treatments were effective in reducing simulated rainwater leachable Pb, with dissolved and total leachable Pb decrease by 15.6–81.9% and 16.3–64.5%, respectively. The PR treatment reduced the total leachable Zn by 16.8%, while TSP and P+T treatments increased Zn leaching by 92.7% and 78.9%, respectively. However, total Cu leaching was elevated by 17.8–178% in all P treatments. Planting promoted the leaching of Pb and Cu by 98.7–127% and 23.5–170%, respectively, especially Pb and Cu in the colloid fraction, whereas the leachable Zn was reduced by 95.3–96.5% due to planting. Concentrations of dissolved leachable Pb showed significantly positive correlation with pH (R2 = 0.827) and negative correlation with dissolved leachable P concentrations (R2 = 0.634), which demonstrated acidic condition was favorable to the immobilization of Pb and implied dissolution-precipitation mechanism of immobilization. In contrast, the concentrations of dissolved leachable Cu and Zn were negatively correlated with pH (R2 = 0.550-0.782) that was mainly attributed to surface complexation/adsorption mechanism by P amendments. There was positive correlation between the contents of Pb and Cu in colloid fraction (R2 = 0.823), indicating synergism of the two during their leaching by acid rain.(2) The pot experiment showed the uptake of various heavy metals in the above-ground parts of two vegetables was as follows: Zn> Cu> Pb. The concentrations of Pb and Cu in the above-ground parts of the plants had a significantly positive correlation (R2 = 0.758), indicating synergism in the uptake of the two metals by plants. In comparison with Chinese cabbage, Chinese kale that was more metal-resistant absorbed much less amount of Pb and Cu, and showed similar uptake of Zn. The concentrations of Pb, Cu and Zn in roots of the two plants were larger than that in the above-ground parts and Chinese kale was more prone to accumulate heavy metals in its root than Chinese cabbage. These results indicated that plants reduce the toxicity of Pb, Cu and Zn by restricting them in their roots via root filtration (e.g. rhizosphere secretion release) or immobilization with exogenous substances. All of the P treatments decreased the concentrations of Pb, Cu and Zn in the above-ground parts of Chinese cabbage by up to 65.1%, 34.3%, and 9.59%, respectively. In the case of metal-resistant Chinese kale, P treatments only decreased Zn concentrations in the above-ground parts by 22.4–28.9%, and showed minor effect on Pb and Cu uptake.(3) The hydroponic experiment showed P treatments failed to reduce the Pb phytoavailability under single Pb stress, which had adverse effect instead. However, under the stress of Pb/Cu/Zn, P-amendments showed significant effect on reducing the phytoavailability of Pb, Cu and Zn. The P+T treatment had the best effect, reducing the phytoavailability of Pb, Cu and Zn by 78.8%, 34.2% and 51.1%, respectively, while the other P treatments presented similar effect. Moreover, the concentration of Pb in nutrient solution was reduced to 0.1 mg L-1 by PR treatment, while it was below the limit of detection due to TSP and P+T treatments. The excellent immobilization effect of P treatments for Pb was attributed to the precipitation of Pb by P amendments in the nutrient solution. The concentrations of Pb and Cu in hydroponic Chinese kale were positively correlated (R2 = 0.794), further indicating synergism of the two metal elements. The transportation ability of heavy metals from the roots to the above-ground parts of Chinese kale under the stress of Pb/Cu/Zn was Zn> Cu> Pb. After P treatments, the transportation ability of Pb under the stress of Pb was stronger than that under the stress of Pb/Cu/Zn due to the difference of Pb concentration and the interaction among Cu, Zn and Pb. In the nutrient solution with single Pb contamination, the DOC and concentrations of Pb were found to have significant positive correlation (R2 = 0.752), thus, the increase of DOC due to rhizosphere secretion would accelerate the mobilization of Pb.(4) The XRD analysis of substances adsorbed on root surface showed the formation of hydroxypyromorphite (or chloropyromorphite) and calcium lead chloride phosphate on the root surface of Chinese kale under the stress of single Pb after TSP treatment, confirming the precipitation immobilization mechanism of Pb. The substance formed on the root surface of Chinese kale under the stress of Pb/Cu/Zn after TSP treatment was mainly porous fluoroapatite that could adsorb heavy metals. Fluoroapatite was also formed dominantly on the root surface of Chinese kale under both the stress of single Pb and Pb/Cu/Zn after PR treatment, indicating the immobilization of Pb by PR was mainly based on the complexation/adsorption mechanism; as small amount of pyromorphite might be formed as well, dissolution-precipitation mechanism of immobilization also played a part in remediation.In summary, plant growing had significant influence on the stability of heavy metals in the P-amended soils, especially the leaching of heavy metals in colloid fraction. Such influence varied with plant species, heavy metals and the different P treatments. The conventional assessment on leaching risks of heavy metals by determining dissolved metals (filtered through 0.45-μm pore size membrane) in leachates could be underestimated since colloid fraction may also contribute to the leaching, and the soil remediation assessment combined with external environmental conditions (e.g. acid rain and plant growing) is definitely excellent way to improve the evaluation system of remediation effect in contaminated soil. Various P-treatments had different effects toward phytoavailability from toward the leachable ability of heavy metals, indicating the immobilization effect of heavy metals in soils highly depended on the evaluation methods. The utilization of hydroponics that avoided interference of soil and XRD analysis could obtain useful information on the interaction among pl ants, heavy metals and P amendments, enabling the investigation on the remediation mechanism of multi-heavy metal contamination in molecular scale.