Phytoremediation Mechanism Of Lead Pollution Base On Organic Acid Exudation From Ryegrass Root

The use of treated and untreated sewage water for irrigation has increased substantially over the past decades in many countries because of the increased scarcity of fresh water. It is well known that sewage is rich in heavy metals, and irrigation with sewage water could therefore result in metal accumulation in soils. Once ending up in soils, the heavy metals not only contaminate soil and water environments, but also enter the food chain via root uptake by plant roots, thereby endangering people health. As a result, removing the heavy metals from contaminated soils has become an urgent issue in many counties. However, since most heavy metals are adsorptive, remediating soils polluted by heavy metals could be difficult and costly. One efficient technique developed over the past two decades is phytoremediation.The principle of phytoremediation of contaminated soils is to use super-accumulative plants to uptake the pollutants. The mechanisms of phytoremediation are complex, involving a number of physical and biochemical processes occurring in both soil and plant. Therefore, it is not surprising that our understanding of the phytoremediation mechanisms is still limited. It has been conjectured that the physical and biochemical processes induced by root exudates in rhizosphere could mobilize the immobile heavy metals, thereby enhancing uptakes by plant roots. But the mechanism associated with this is not well understood. We hypothesized that the root exudates released by super-accumulative plants into rhizosphere decrease the pH value of soil water; a decrease in pH value could enhance metal desorption, thereby mobilizing the adsorbed metals for plant roots to uptake. To test this hypothesis, we took the phytoremediation of lead by ryegrass roots as an example. A series of experiments were carried out in attempts to understand 1) if a change in pH value increases lead desorption, and 2) if the root exudates of ryegrass results in a change in pH and by how much, 3) the effect of lead concentration on the amount of root exudates released by roots and the associated root uptake.This thesis consists of nine chapters and is organized as follows. We first measured the adsorption and desorption of the Pb2 + under different pH value (3, 5, 7, 9, 11) and the various organic acids (EDTA, oxalic acid, tartaric acid, acetic acid, malonic acid, malic acid and citric acid); the Tissier method was used to study the effect of various pH values on the form of Pb²⁺ in soils. We then experimentally investigated the efficiency of uptake of Pb²⁺ by the roots of ryegrass under Pb²⁺ concentration ranging from 10 mg/l to 1000mg/l; in the meantime, we also analyzed the organic acids exudated by ryegrass roots under various Pb²⁺ concentrations. To see if the root uptake of Pb²⁺ can be engineered, we externally added organic acids (EDTA, oxalic acid, acetic acid, malonic acid, tartaric acid, malic acid, citric acid) at various concentrations (0.1,0.5,1,2,3 mmol/l) into the solution where the ryegrass were grown. Finally, the uptake of Pb²⁺ by ryegrass roots from soil was studied. The seedling of ryegrass were grown in pots packed with soils under various Pb²⁺ concentrations (0,100,300,500,1000mg/kg) and externally added organic acids (EDTA, oxalic acid, acetic acid, malonic acid, tartaric acid, malic acid) at concentration from 1to 7mmol/kg was investigated. Mesh was used to separate the rhizopshere from the bulk soils. Results obtained from the experiments are summarized as follows.(1) The adsorption-desorption of Pb²⁺ under various pH values indicated that pH value have a significant impact on adsorption-desorption. The increasing amplitude of adsorption capacity is much larger when the pH value less than 7. The adsorption isotherm can be described by a nonlinear model ln(S)=K1×CK2, where S is the adsorbed Pb²⁺ and C is Pb²⁺ concentration in water, K1 and K2 are fitting parameters. The desorption quantity decreased with pH value, and increased with absorption capacity. And it is evident that the added organic acid improved the mobility of Pb²⁺. The order of the acid follows EDTA>citric acid> malic acid> malonic acid> acetic acid>tartaric acid>oxalic acid in terms of their impact on Pb²⁺ desorption.(2) The results on the form of Pb²⁺ under different pH value showed that when the pH value is less 7, Pb²⁺ is more bio-available if its concentration is in the range 10-100mg/l. The bio-availability decreases when concentration increases from 200 to1000 mg/l. The proportion of Fe-Mn oxides fraction is the maximum when Pb²⁺ concentration was in the range of 0-100mg/l. Residual fraction is much larger than other forms in the range of 200-1000mg/l.(3) The results measured under water culture condition and various Pb²⁺ concentrations indicated that, at concentration of 400mg/l is the inflection point to the sensitivity of organic acids to pH values, and 200mg/l is the inflection point of plant height, dry matter quantity and root tolerance index. Pb²⁺ content measured in ryegrass increased with the Pb²⁺ concentration in the nutrient solution, but the rate decreased as Pb²⁺ concentration increased. Organic acid exudation by the ryegrass roots is mainly oxalic acid and acetic acid at initial stage, but as the plants grow, they became dominated by oxalic acid, tartaric acid, malic acid, acetic acid and citric acid. The solution becomes toxic when Pb²⁺ concentration in nutrient solution was over than 800mg/l.(4) The effect of adding exogenous organic acids on the uptake of Pb²⁺ by plant roots grown in solution culture indicated that, organic acid increased the dry matter in both shoot and root. Adding EDTA enhanced the transfer of Pb²⁺ from soil to shoot, but the increase in biomass is limited. Malonic acid (from 1 to 3mmol/l), tartaric acid can increased the content of Pb²⁺ in shoot, and Acetic acid (from 1 to 3mmol/l) increased the biomass of and the content of Pb²⁺ in roots. In terms of the effect of different organic acids on absorption and transfer of Pb²⁺ in static water, tartaric acid gave the maximum when at concentration of 0.1mmol/l, malonic acid gave the maximum when at concentration of 0.5mmol/l, and acetic acid gave the maximum when at the concentration in the range of 1-3mmol/l.(5) When grown in soil pots under different Pb²⁺ concentration, the main organic acids exudated by the ryegrass roots were found to be oxalic acid, malic acid and acetic acid. The pH value, Eh and Pb²⁺ content measured from soil taken from rhizosphere were less than that in bulk soil. The Pb²⁺ content measured in plant was directly related to Pb²⁺ concentration in soil, and increased as plan grows. The uptake of Pb²⁺ by ryegrass is the highest 40-50 days after the seedling and when Pb²⁺ concentration is in the range from 300 mg/kg to 500mg/kg. The adsorption capacity (rate) of soil in rhizosphere is slightly larger than bulk soil when the Pb²⁺ concentration is in the range of 200-1000mg/l, and the difference increases with growth stage. The rhizosphere of ryegrass can improve the proportion of exchangeable fraction and organic fraction of Pb²⁺. The bioavailability rate of Pb²⁺ in rhizosphere was higher than that in non-rhizosphere, and the difference decreased as the Pb²⁺ concentration in soil decreased. The bioavailability rate of Pb²⁺ in both rhizosphere and non-rhizosphere increased as plant grow.(6) The measurements by adding exogenous organic acids on the uptake of Pb²⁺ by ryegrass roots in soil indicated that, the remediation effect of EDTA was significant but has to be used with car as it could lead to a secondary pollution. Malic acid as one of organic acids exudated by ryegrass root could also increase the content of Pb²⁺ in the shoots. The Pb²⁺extraction amount of acetic acid treatment is more than other organic acid. And acetic acid can enhance phytoremediation effect. And it reached the maximum when concentration was at 3mmol/kg, then decreased gradually. And acetic acid could enhance the biomass.