The distribution and retardation coefficients as a tool in selection of low-cost sorbent as a material for permeable reactive barrier – SEM-EDS analysis

Abstract

Introduction A permeable reactive barrier (PRB) is considered as an innovative, green engineering approach for remediation of contaminated groundwater, using a passive, in situ technology with high potential of treatment of contaminant plume at in a cost-effective manner. Recently, scientific investigations are focused on discovering of materials for PRB which are abundant in nature or are waste material from another process. In this study, distribution and retardation coefficient were used as a tool to select various natural materials such as natural zeolites, clays and apatite as a material for PRB for removal of heavy metals lead, cadmium, zinc and cooper from contaminated groundwater. In addition, scanning electron microscopy (SEM) analysis and semi-quantitative energy dispersive spectrometry (EDS) of selected sorbent were analysed before and after saturation with the best removing metals. Experimental Methods The following sorbents were investigated: natural zeolite, NZ (Zlatokop deposit,Vranjska Banja, Serbia), IMZ (prepared by modification of natural zeolite), bentonite (Šipovo deposit, Bosnia), apatite (ore deposit Lisina, Bosilegrad, Serbia), concentrate apatite (prepared from apatite), kaolin (plant for production of quartz sand, Rgotina Serbia) (Ugrina et al., 2017). All samples were analysed on sorbent density and sorbent porosity (results not given here). Batch sorption experiments are performed by mixing metal ion (Pb, Cu, Zn or Cd) solutions with each sorbent, at solid/liquid ratio of 10 g/l, during 48 hours at room temperature. The leaching experiment included evaluation of metal released from saturated sorbents in ultrapure water with adjusted pH values in range 6.07-6.47. SEM-EDS of selected sorbent were analysed on a JEOL JSM-6610 instrument. Results and Discussion Distribution coefficient Kd (l/g) and the retardation coefficient Rd (-), are calculated using the following equations:

where co and ce are the initial and equilibrium metal concentrations (mmol/l), V is the metal solution volume (l), m is the sorbent mass (g),  is the sorbent density (g/cm3),  is the sorbent porosity (-). Figure 1 represents the results of retardation coefficient. Based on calculated distribution and retardation coefficients using equation (1) and (2), raw bentonite and IMZ showed the highest removal potential for zinc, cadmium and lead, while raw bentonite was the best option for cooper removal, as in system of Cu-IMZ, adjustment of pH was necessary in order to avoid precipitation. Beside the distribution and retardation coefficients, the leaching test in ultrapure water solution at pH=6.07-6.46 was performed and leaching of lead and cadmium from raw bentonite was noticed. This indicates that distribution and retardation coefficients are not sufficient for selection of material for PRB, as secondary pollution as a consequence of leaching cannot be predicted. Thus, based on retardation coefficients and performing leaching test, selected low-cost sorbents with the best removal properties for investigated heavy metals are: raw bentonite and IMZ for zinc ions, raw bentonite for cooper, and IMZ for lead and cadmium. SEM-EDS analysis is giving insight into surface characterization of selected sorbents before and after saturation with certain heavy metals (Figure 2.).