Effect of Bacterial Surfactant along with Application of Chelators on Soluble Cadmium in a Calcareous Soil

 Surfactants as surface-active substances with combined hydrophobic and hydrophilic properties are widely used in various fields. In soil remediation processes these substances can be used to increase the availability of organic and inorganic contaminants to improve microbial decomposition of organic pollutants or heavy metals adsorption. In recent years, researchers have been seeking to produce and use surfactants that are more environment friendly. In this regard, produced biosurfactants by microorganisms are of special importance due to their environmental benefits. Microorganisms produce a wide range of biosurfactants. Biosurfactants are extracellular compounds that can combine with metals such as zinc, copper, and cadmium and can increase the solubility of these metals and reduce their toxicity. Negatively charged anionic biosurfactants such as rhamnolipids and lipopeptides can increase heavy metals availability by combining to metals and changing the properties of soil solution. In this study, the effect of surfactant application from Pseudomonas putida and Bacillus subtilis and some chelators include sodium citrate, humic acid and Na2-EDTA on soluble cadmium in a contaminated calcareous soil was investigated.

This study was conducted as factorial in a completely randomized design in laboratory conditions at several steps separately. A calcareous soil sample was contaminated with 15 mg kg-1 cadmium from the source of Cd (NO3)2. Contaminated soil incubated for 4 weeks at field capacity. Acid deposition method was used for surfactant extraction from culture medium of Pseudomonas putida KT-2440 and Bacillus subtilis 1795. The structure of extracted biosurfactants was investigated by FTIR. Equilibrium time was obtained by determining the amount of soluble cadmium at times 6, 12, 24, 36, 72 hours by adding 1mM sodium citrate, humic acid and Na2-EDTA to the contaminated soil (ratio of 1 to 5 soil to solution).The concentrations of 0, 0.1, 0.25, 0.5, 1 and 2 mM of humic acid, sodium citrate and Na2-EDTA were used to determine the appropriate concentration of each chelator. To investigate the interaction of chelators and biosurfactants on soluble cadmium, an experimental was conducted as a completely randomized design with factorial arrangement design. Experimental treatments consisted of three types of chelating agents (sodium citrate, humic acid, Na2-EDTA and control), two types of surfactants from Pseudomonas putida and Bacillus subtilis, and five concentration levels of the biosurfactants (0, 25, 50, 100 mg L-1).

The highest amount of soluble cadmium (11.59 mg L-1) was observed in Na2-EDTA treatment at 72 hours, which was significant compared to the other treatments. The lowest amount of soluble cadmium was obtained through application of sodium citrate (0.205 mg L-1) at 36 hours. In all studied concentrations, Na2-EDTA had the greatest effect and sodium citrate had the least effect on soluble cadmium. While the use of Na2-EDTA at all concentrations caused a significant increase in soluble cadmium, sodium citrate had no significant effect on soluble cadmium at studied concentrations. Humic acid at concentrations higher than 0.5 mM significantly increased the soluble cadmium. Increasing the concentration of humic acid and citrate from 1 to 2 mM did not show any significant impact on soluble cadmium. At all levels of biosurfactant application, Na2-EDTA and humic acid caused a significant increase in soluble cadmium concentration. In control and sodium citrate treatments, application of biosurfactants did not cause significant difference in the concentration of soluble cadmium. The highest amount of soluble cadmium was obtained as a result of the application of Bacillus subtilis surfactant and Na2-EDTA. However, increasing the concentration of Bacillus subtilis surfactant from 25 to 100 mg L-1 had no significant effect on increasing the efficiency of Na2-EDTA.  Pseudomonas putida surfactant had no significant effect on soluble cadmium in Na2-EDTA application. While in humic acid treatment, the application of the Pseudomonas putida surfactant at the highest concentration (100 mg L-1) increased the concentration of soluble cadmium. Using Bacillus subtilis surfactant did not have effect on soluble cadmium in application of humic acid.

Among the studied chelators (sodium citrate, humic acid and Na2-EDTA), Na2-EDTA had the greatest effect on soluble cadmium. While sodium citrate had no significant effect on soluble cadmium. Surfactants from Pseudomonas putida and Bacillus subtilis had different effects on increasing the efficiency of studied chelators and soluble cadmium in the studied soil. In Na2-EDTA and humic acid application, surfactant from Bacillus subtilis at a concentration of 25 mg L-1 and surfactant produced by Pseudomonas putida at a concentration of 100 mg L-1 had a significant effect on soluble cadmium, respectively. It seems using biosurfactants and chelators on increasing soluble cadmium in soil can be useful for phytoremediation purposes to increase its uptake by plant. However, further research is needed.