Effects Of Surfactants On Phytoaccumulation Of Heavy Metals From Soil

It has been more than one hundred years since surfactants were used as the main substance in soap. The usage of surfactants has been widen to all the production and economic technology fields, such as mining, foods, agriculture, forest, transportation, environment protection and drugs. Of those surfactants, most of them have been discharged into the environment. Due to the difficulty to be degraded, they were pollutants, however, they were used to remedy the polluted environments. The surfactant-enhanced remediation techniques based on the inclusion and have been successfully used in remedy soils polluted by some organic pollutants that were difficulty to be dissolved, degraded and reused since 1990s. Recently, there are some researches related to the use of surfactant in remedy the heavy metal polluted environment, however, little information is available on the use of surfactants in ameliorating soils polluted by heavy metals.Based on the relationship between surfactant, heavy metals and soil- plant system, successive sorption experiment, simulation experiments, column leaching experiments and plant culture experiments were carried out to investigate the enhanced effects of the combination of surfactants with chelates on phyto- remediation of Cd polluted soils. The experiment conditions included the type, dose, time and methods of applications. The mechanisms of soil physico-chemistry, plant physio-chemistry and soil-plant coupling were studied and the environment safety from the secondary pollution of surfactant application to soil-water system was also examined. The results cannot only rich the fields of the pollution eco-chemistry of surfactants, their environment behaviour and effect, but also establish a surfactant-enhanced phyto-remediation technique.Pot experiments with orthogonal design were conducted to investigate the effect of types, application time and dose of surfactant, the addition time, dose of EDTA, species and concentrations of heavy metals and plant species on the growth, heavy metal content, uptake and their distribution in plants. The results showed that plant species, heavy metal species and the types of surfactants produced more influence than other factors. Zea Mays, which is a C₄ plant, has higher biomass, while Brassica juncea var. multiceps, which is a C₃ plant, was easier to accumulate heavy metals. The enhanced effects of the combination of surfactant and EDTA were cadmium > copper > lead, with the average concentrated indexes of 70.31%, 67.42% and 28.92%, respectively. In the case of surfactant, the anionic-type and non-ionic type surfactants were more effective in enhancing the effects of phyto-remediation. Considering the cost and the results, the anionic-type SDS and the nonionic type Tween 80 were recommended.The main mechanisms of surfactants and EDTA in enhancing heavy metal uptake by plantswere that they increased the phyto-availability of heavy metals in soils and the transportation of heavy metals from root to shoot. The chlorophyll content of surfactant and EDTA treated plants decreased gradually with the progress of time and cadmium accumulation. The role of surfactant and EDTA in membrane resulted in more cadmium entering into plant, thus cadmium concentration in plant increased and the lipid peroxidation occurred in plant cell, then MDA content improved. The increase of cadmium accumulated in stem and leaf fitted Logistic equation. When the plants were harvested at the 56 days, 80% of the theoretical maximum adsorption can be achieved.Germination experiments showed that the type and application rate of surfactants had no obvious influence on the germination rate, while had big influence on germinating vigor. The inhibition effects of surfactants were CTAB>TX100>SLS, and when cadmium was added, the inhibition effects became more serious. With the increasing of SLS concentration, the relative water adsorption rate and the relative electric conductivity of seed decreased. This further illustrated the feasibility of use the anionic surfactant SLS to enhance cadmium uptake by plants.Typical investigation and sampling analysis showed that the average contents of SAS and cadmium in the main agricultural lands in Chongqing area were 0.71⁷.95mg kg'1 and 0.01⁰.74mg kg"1, respectively. The SAS in upland soils was much higher that that of paddy soils, with the average content of 4.34 mg kg'1 and 3.89 mg kg"1, respectively. The SAS in soils showed normal distribution, with most in the range of 2.0⁴.0mg kg"1, which involved in 46.12% of the upland soil and 50.0% of the paddy soils. The cadmium contents showed skewed distribution, the centre zones in upland soils and paddy soils were 0.2⁰.4 mg kg"1 and 0.3⁰.5 mg kg'1, respectively, and 52.63% of upland soil and 42.10% of paddy soils followed in the ranges.The successive batch experiments showed that the nonionic, cationic, anionic surfactants and EDTA treatments significantly improved the dissolution speed and amount of cadmium from purple soils, especially when surfactant and EDTA were combined. The compound effects of surfactant and EDTA on cadmium dissolution were several ten times higher than those of surfactant or EDTA used alone and increased with the increase of surfactant or EDTA in balance solution.Sorption-desorption experiments showed that when the purple soils were treated with SAA, EDTA and SAA+EDTA, the kinetics of cadmium desorption fitted the two-constant equations best, SAA/EDTA promoted cadmium desorption from purple soils, their effects were in accordance with their ability to dissolve cadmium. Different surfactant treatments resulted in big difference in the chemical mechanism of cadmium sorption by soils, this might be related to their type and application * rates. The effects of DBS, CTAB, TX100, EDTA, EDTA/DBS, EDTA/CTAB, EDTA/TX100 on cadmium sorption bv the ourDle soils were well described bv Freundlich equations, the sorcrtionconstant of k;n were almost the same for soils added with DBS;CTAB;TX100;this implied that their sorption was not different from the control. The k of EDTA;EDTA/DBS;EDTA/CTAB and EDTA/TX100 added soils was obviously lower that that of control;while the n was higher;this showed that their sorption was very different from that of the control.The simulation experiments found that the degradation of SDS in soils obeyed the one-grade reaction kinetic equations. Cadmium decreased the reaction constants;the reaction constants were high related to the properties of both SAS and soils. When used in pytoremediation of heavy metal polluted soils;over 90% of SDS would be degraded during one growing season (about 60 days);therefore;it did almost no harms in environment.Leaching and shaking experiments found that the effect of heavy metal removal from sewage sludge was citric acid>EDTA>SDS. In leaching experiments;antagonistic effects existed between citric acid and EDTA on removal of Cdr;while in shaking experiment;synergistic effects existed;synergistic interaction on the removal cadmium in leaching experiments was observed;while antagonistic interaction in extraction Cd from sludge. Considering saving N and the cost;the recommended method to removal Cd from sludge was 0.3 mol L'1 citric acid;then 0.02 mol L"1 SDS;with the sludge to solution ratios of 1:10 and shaking time of 1 hour (totally 2 hours). At this condition;63.71% of cadmium in sludge was removed while the loss of N was only 12.30%.Due to the limitation of experiment conditions;there are many topics needed to be studied further;for example;the soil chemical mechanism;plant physio-biochemical mechanism that surfactant and EDTA enhance the absorption of heavy metals by plants;and their risks in eco-environment. At the same time;the results of research should be put into practice as soon as possible.