Effect Of Citric Acid And Oxalic Acid On Bioactivity Of Lead In Soils Of Tea Garden

China, the origin of tea, is a big country of production and trade of tea, the tea industry has taken an important role in the national economy and international trade. In recent years, with the rapid development of China’s tea industry, safety and quality problems in tea has cause for wide concern and attention by consumers and scholars at home and abroad, what’s more, tea lead pollution has been a serious problem. As everyone knows, lead is the physiological and neurological toxin, and it will affect almost all of the human body’s vital organs and systems, and the lead in tea leaves was mainly uptake from the soil by root. Therefore, it is very important for the sustainable development of Chinese tea industry and the health of consumers to study speciation of lead transformed, lead pollution and its prevention in soil of tea garden.Tea tree can secrete citric acid and oxalic acid to the soil in its natural growth process, cause soil acidification, decreases the soil pH value, increases the solubility of some heavy metal ions, to a certain extent, it is harmful to the growth of tea. Therefore, the study on the design of3different pH of soil,0and180mg/kg soil lead pollution level,2kinds of imported acid type which is citric acid and oxalic acid,5levels of imported acid concentration which is0.1,0.2,0.3,0.4,0.5mmol/kg, after50days in culture, discuss and analysis the effect of citric acid and oxalic acid on the changing of soil pH, organic matter, organic acid, and the effect on the bioavailability of lead. The main results obtained are as follows:(1) From pH3.5to pH4.5to pH5.5, soil water soluble and exchangeable Pb content showed declining trend, carbonate bound Pb was rising, Fe-Mn oxides and organic matter content of lead is first decreased and then increased, while the residual lead is increased then drop. In pH=4.50, the soluble Pb and exchangeable Pb content is relatively less in tea, but the residual lead is more.(2) The order of all kinds of lead in the original soil is the residual> organic matter>Fe-Mn oxides> carbonate bound> exchangeable state> water soluble. It is visible that Pb of residual form is the main form in soil, this form of lead can exist in the sediments of long-term stable under normal conditions, and it is not easy to release, not easily absorbed by plants, thus increasing the water soluble exchangeable Pb to transform to the residual lead can reduce the harm of heavy metal lead. But when spraying180mg/kg lead in soil, the content of various forms of lead in soil has increased, and exchangeable Pb is in the largest increase, carbonate Pb the smallest increase, it tells that when the lead content of heavy metals in soil is more, it will appear in the form of exchangeable Pb. The increase is arranged from large to small is exchangeable state> water soluble> organic matter> residual> Fe-Mn oxides>carbonate bound. The change trend is same to the original tea garden soil, but it is more obvious and significantly.(3)At the beginning, It is a linear relationship between the bioavailability of lead and soil pH with the changes of soil pH. But when the number of soil pH is more than4.50, water soluble lead and exchangeable lead content does not comply with this linear relationship, and to slow the trend. Comprehensive trend as a whole, in fact, soil pH and the water soluble lead content rendering is a logarithmic relationship, that the initial change more strongly, with the increase of pH, water-soluble lead and exchangeable lead content reduced soon, but when pH rise to4.5, both reduced slowly. When the pH is4.64, while water-soluble lead remains largely unchanged as the pH increasing, when the pH is4.54, exchangeable lead too. It shows that there is a threshold of transition to the content of water soluble lead and exchangeable lead in soil, the pH value of this transition is about4.50.(4) The correlation between the levels of water-soluble lead and organic matter is not different from exchangeable Pb, it is the negative correlation at first, and it turns into positive correlation after20days’culture. The correlation between the levels of water-soluble lead and organic acids is not different from exchangeable Pb too, it is the negative correlation at first, and it turns into positive correlation after30days’culture. To a certain extent, the content of organic matter and organic acid in soil reflects the content of water soluble and exchangeable Pb in the latter part of the culture.(5) To the reducing effects of bio-availability Pb, oxalate acid effects is better than citric acid. To carbonate bound Pb and Fe-Mn oxides Pb, the content increase when a certain amount of imported acid is added to the soil. To organic matter lead, citric acid increase but oxalate acid decrease. But the content of residual lead decreases when both oxalate acid and citric acid is added to the soil.No matter the soil is contaminated by lead or not, entering a low concentration of citrate acid and oxalate acid will reduce lead in soil-water-soluble and exchangeable lead content, and when the higher the concentration of Pb in soil is, the effect of reducing is more obviously. At the same level, the reducing effect of oxalate acid is better than citrate acid, but not obviously. But high concentration would increase their content. To water soluble Pb, the line of demarcation is0.3mmol/kg of both citrate acid and oxalate acid. But to exchangeable Pb, the line of citrate acid is0.2mmol/kg, and the line of oxalate acid is0.3mmol/kg.