Study On The Removal Of Heavy Metals From Soils By Surfactants

Heavy metals residing in the environment can be extremely hazardous. Remediation of heavy-metal contamination has become an important and urgent issue. Chemical leaching and flushing/washing remediation is an important technique of contaminated soil remediation. While choosing a proper flushing solution is the key factor of this method.Batch experiments were conducted in this study to evaluate the performances of two kinds of surfactants, saponin and alkyl polyglycoside (APG) for removal of heavy metals from contaminated soils. The effects of concentration, pH values and ionic strength of surfactant solution on the desorptions of metals from soils by surfactant washing were investigated. The speciations of heavy metals in the soils were identified before and after surfactants treatment. The complexation of cadmium ion with saponin was confirmed by Fourier transform infrared spectroscopy (FTIR). The conditional stability constants (K) for each of metal complexations with saponin were determined using an ion-exchange resin technique.It was found that higher concentration and lower pH value of saponin solution contributed to a better removal efficiency of heavy metals. The maximum removal efficiency of cadmium, copper, zinc and lead reached to 45.6%,24.4%,19.0% and 17.6%, respectively, by a 50 g·L-1 saponin solution at pH 5.2. Higher concentration of APG solution and the condition of pH value about 5 contributed to a better removal efficiency of heavy metals. The maximum removal efficiency of cadmium, copper, lead and zinc reached to 77.7%,40.5%,24.5% and 20.0%, respectively, by a 85g·L-1 APG solution at pH 5.2. The concentration of NaCl solution had a positive influence on desorption of Cd, but almost no efforts on those of Cu, Zn and Pb. Comparing the speciations transformation of heavy metals in the soils before and after surfactants treatment, the results showed that saponin and APG had different efficiency on the removal of heavy metals which exist in different speciation in soils, and heavy metals in exchangeable and carbonated fractions could be more easily washed away by both saponin and APG.The complexation of cadmium ion with saponin was confirmed by Fourier transform infrared spectroscopy (FTIR). The conditional stability constants (K) for each of metal complexations with saponin were determined using an ion-exchange resin technique. The order of measured stability constants follows:Cu>Zn>Cd>Pb, andthe values of lgK ranged from 3.91 to 6.60. The complexation capacity of saponin with metal ions (Cd, Pb, Zn) was approximately 1:1 on a molar basis except for Cu ion (2:1). Moreover, the mobilization of cadmium ion was found to be positively correlated with those of other heavy metals. These heavy metals might be removed from soils by forming soluble complexes with saponin or be transferred into aqueous saponin phase through metal-bridging mechanism.Using saponin and APG as flushing solution can remove heavy metals from soils effectively. The toxicity and bio-reusability of the heavy metals in soil could be significantly decreased after saponin or APG washing.