Effect Of γ-Polyglutamic Acid-Activated Phosphate Rock On Different Forms Of Pb In Pb-Contaminated Soil And Pakchoi Growth

Heavy-metal pollution is the primary type of soil contamination. In China, there 20 million hectares cultivated land have been polluted by heavy metals that impaired our crop yield as well as food safty, putting ecological environment and human's health into vital danger. Heavy metals will bring their harm to human being as they enter into food chain through agricultural food production after contaminating farmland. Therefore, high effective remedial technique has to be built up urgently.Since repairing heavy-metal polluted soil has been being one of the hot issues in environmental research field at home and abroad, many reparing techniques for heavy-metal polluted soil have come into being already, including physical techniques, engineering measures, biological ways as well as chemical inactivation method etc.. A great amount of studies covering soil heavy metals' immoblization by phosphate rock has been working on, meanwhile, using y-polyglutamic acid (y-PGA) as a treat-agent, which reacted with specific heavy-metal contaminants in water attaining remedial goal for heavy-metal pollution, has intrigued broad concern. However, any research about their joint effect on immoblization of heavy metals in cultivated soil has not been seen yet so far.In this experiment, y-PGA was used to activate phosphate rock and then the effect of y-PGA-activated phosphate rock on the soil basic chemical properties and content of different forms of Pb in Pb-contaminated soil were studied. Moreover, in order to prove repairing effect of y-PGA-activated phosphate rock on heavy-metal contamination, Pb content of different parts of harvested pakchoi as well as its yield were determined and measured after pot experiment with the same soil, in hope of laying the root for further research about immoblizing mechanism of y-PGA-activated phosphate rock on heavy metals in soil as well as related techniques afterward. As a result, the development of stable and effective heavy-metal-immoblizing materials likeγ-PGA-activated phosphate rock for repairing heavy-metal polluted farmland would be supported thereotically and technically. The experimental results showed as follows:1.It was showed that Pb dissolution strengthened by y-PGA with its concentration augmenting within the range of 0-20g/L. The highest concentration of 20g/L came to the highest rate of 93.2% Pb dissolution. As a result, regarding to the effect on pakchoi growth, y-PGA played the role of enhancing absorption of Pb to inhibit the growth of pakchoi by reducing fresh weight of it.2. y-PGA promoted P-releasing of PR showing its function of PR activation. Along with the concentration rising of y-PGA within 0-20g/L, the water-soluble P of BPR and NPR was promoted to release increasingly, reaching 0.01-0.41g/kg and 0.01-0.45g/kg. Meanwhile, pH of extractant (water) was rising with the y-PGA concentration increasing from 0 to 10.0g/L.3.To some extent, PR demonstrated immobilizing function for Pb in soil, however, the effect was far less weak than that brought by activated BPR and NPR. The result showed that, with y-PGA concentration rising within 0-20g/L, y-PGA-activated BPR and NPR respectively raised residual form Pb content by 200% and 150% around. Apparently, the application of activated BPR and NPR can lower Pb mobility in soil so as to attain the effect of immobilizing Pb. At different time within 40 days after activated PR was applied, the soil basic chemical properties were determined and compared with the counterparts brought by inactivated ones. It was showed that activated BPR and NPR raised soil pH around 0.29 and 0.38 more than that caused by inactivated ones respectively. For soil CEC, the increase number fell on 7.4～11.9% and 12.4～29.4% more for activated BPR and NPR respectively; For Ca2+ content of soil, the growth range came to 5.3～8.2% and 3.4～5.6% more for activated BPR and NPR respectively; For soil available P, the rise range reached up to 3.16～14.5mg/kg and 4.80-8.38mg/kg more for activated BPR and NPR respectively.4.Application of BPR and NPR brought pakchoi production increase by 6.7% and 10.4% respectively, however, y-PGA-activated BPR and NPR contributed to the increase of it much more. With y-PGA concentration rising within 0～20g/L, y-PGA-activated BPR and NPR respectively raised pakchoi fresh weight 23.6% and 19.7% more than inactivated ones. Application of BPR and NPR slightly reduced Pb absorption of pakchoi leaves up to 3.92mg/kg and 3.53mg/kg respectively comparing with the control.However, y-PGA-activated BPR and NPR's application resulted in far more decrease of Pb content of pakchoi leaves, being positively-related with the concentration of y-PGA. The best effect was attributable to 20.0g/L y-PGA-activated BPR and NPR which led to 13.04mg/kg and 12.25mg/kg P content decrease respectively. The same outcome attributed to y-PGA-activated BPR and NPR came to pakchoi roots but the only difference reflected in Pb absorption content. Application of BPR and NPR slightly reduced Pb absorption of pakchoi roots up to 21.12mg/kg and 11.36mg/kg respectively comparing with control. But y-PGA-activated BPR and NPR's application resulted in far more decrease of Pb absorption for pakchoi roots, being positively-related with theγ-PGA concentration. The best effect of P content decrease was brought by 20.0g/Lγ-PGA-activated BPR and NPR up to 24.58mg/kg and 28.14mg/kg respectively.All in all, y-PGA played a role in P releasing to some extent, bringing phosphate rock activation effect into practice. With y-PGA rising from 0 to 20.0g/L, the pH of extractant solution went up as the water-soluable phosphate volume surged gradually, contributing to the residual Pb content rise by 200% and 150% comparing with the treatments of sole BPR and NPR application separately, thereby promoting Pb immoblization induced by phosphate rock. Besides, y-PGA-activated BPR and NPR were able to lower Pb content in leaves and roots of Pakchoi. Meanwhile, comparing with the enhanced pakchoi yield up to 6.7% and 10.4% by sole BPR and NPR application respectively, it reached to 23.6% and 19.7% by the same treatments but combined with y-PGA, indicating y-PGA could further improve phosphate rock's ability of raising pakchoi yield.