The Role of polyacrylamide polymer in retention of lead heavy metal contaminant

In recent years the use of nano materials in engineering projects has significantly increased. In fact, the impact of nano materials as a component in composite materials is one of the new horizons in engineering science. On the other hand, existence of heavy metal contaminated soils is one of the common problems in geo-environmental projects all around the world. In the process of retention of heavy metals by clayey soils, the pH of soil solution plays a significant role. In fact, an increase in pH of soil pore fluid causes a noticeable increase in contaminant retention. On the other hand, the use of additives in soil can increase the contaminant retention as well. In comparison with other additives such as cement, polymers do not require a long curing conditions. In addition, they have a positive impact on permeability of compacted soil, in which their presence decrease the soil permeability. In spite of several researches on the interaction process of clay minerals and polymer, there are very limited researches on the interaction process of polymer-clay minerals-heavy metal contaminant. Therefore, the main objective of this paper is to investigate the role of polyacrylamide polymer in retention of heavy metals in bentonite. To achieve the above mentioned objective, at the first step, the buffering capacity of polymer treated bentonite samples were measured. In this series of experiments, different concentrations of nitric acid from 0.002 to 0.02 molar were prepared. Then, 4 grams of bentonite and bentonite treated with different concentrations of polymer were poured in centrifuged tubes. Then, 40 cc of nitric acid were added to each centrifuge tube. After equilibrium period, the pH of soil suspension was measured and reported. In the second step of this research, two different series of experiments were performed. In the first series of experiments of this research, bentonite treated by polymer were exposed to different concentrations of heavy metal contaminants. In the second series of experiments, contaminated bentonite samples were treated by different percentages of polymer. The contaminant retention of these samples was investigated by performance of sets of batch equilibrium experiments. The achieved results indicate that the polymer treated bentonite sample (with 3% polymer), after exposure to 200 cmol/kg-soil has shown 19% increase in contaminant retention in comparison to bentonite sample. However, the addition of 3% polymer to contaminated bentonite with 200 cmol/kg-soil lead nitrate has shown 72% increase in contaminant retention in comparison to contaminant retention of bentonite sample. Based on the achieved experimental results it is concluded that there are three phases in heavy metal contaminants in interaction process of bentonite-polymer-heavy metal. These phases include retention by double-layer of clay, the contaminant retention in micro pores of clay minerals which are solidified by polymer, and the contaminate retention capability of polymer. According to the achieved results, the solidification effect of polymer has more contribution to the contaminant retention than the polymer buffering capacity. This proves that post-solidification of contaminated bentonite is a practical method for efficient prevention of contaminant transport in clayey soils.