Selenium Removal from Sulfate-Containing Groundwater Using Granular Layered Double Hydroxide Materials

Abstract

Due to recent changes in regulatory discharge limits, selenium removal from electric utility wastewaters is becoming an important issue. In this work, Mg–Al–CO3 layered double hydroxide (LDH) in granular form was evaluated for treatment of selenium-containing groundwater in small scale column tests. The LDH material showed good capacity for removing selenate from groundwaters that contained very high levels of sulfate and total dissolved solids. Column tests were investigated using the granular LDH to treat groundwater containing trace levels of selenium <2 ppb. Removal of sulfate using chemical pretreatment of the groundwater resulted in about 3× higher selenium loading onto the granular LDH. The structural changes in the media after exhaustion and analysis of the effluent water from the column test were also studied to better understand the species removed by the LDH. These results show that the LDH is a promising sorbent for removing selenium from wastewaters with high levels of sulfate and background species.

Layered Double Hydroxide Sorbents for Removal of Selenium from Power Plant Wastewaters

“…After calcination at 500 • C, the powder (referred to as LDH-500C) could be easily dispersed into water and displayed the mixed metal oxide structure ( Figures S1 and S2). Hence, LDH-500C could be used to capture anions using the memory effect discussed above, in addition to surface adsorption and anion-exchange [44]. The reconstruction of the layered structure via the memory effect once LDH-500C was exposed to water solutions containing selenate was also confirmed ( Figure S1).…”

“…The physical properties of the granular LDH used in the column tests of this study were already investigated in our prior work [44,46], which established that the as-obtained, mixed metal oxide material exhibited the cubic periclase structure of a magnesium/aluminum solid solution, which is typically obtained when LDH is calcined at high temperatures to remove interlayer water and anions [47][48][49]. Upon re-introduction of the granules to DI water, the layered double hydroxide structure was successfully recovered [44] in what is known as the "memory effect" of LDHs [38]. This memory effect can also be exploited to remove anions from water, as they are captured by the LDH when the layered structure is reconstructed.…”

“…The equilibrium adsorption isotherms showed a good fit to the Langmuir model ( Figure 1, Supplementary Materials); the calculated maximum capacity (Q max ) and the equilibrium constant were 66 ± 0.65 mg/g and 0.14 L/mg, respectively. Given that LDH-500C was calcined, the actual adsorption capacity of the non-calcined layered media used in the column tests was likely around 30 mg/g, on the basis of previous reports showing selenium sorption on non-calcined LDH to be about half that of the calcined form [38,44] and assuming no decrease in adsorption capacity due to granulation (justified on account of the similar surface area, i.e., 188 m 2 /g for the granules vs. 179 m 2 /g for the calcined powder). Due to differences in sorbent dosages and pH conditions used in isotherm experiments, it is not straightforward to compare the Q max values obtained here to others from the literature.…”

“…The breakthrough curve for the column test conducted on the as-obtained CT blowdown water (initial total [Se] = 2.67 ppb and [S] = 187 ppm) and the results are shown in Figure 3 and Table 1, respectively. In Table S2, these results are further compared with those obtained from a small-scale column test performed using the same LDH granular media to remove Se from the makeup water (groundwater) used to supply the cooling tower in our previous study [44]. The adsorption capacity of the LDH in the CT blowdown was determined to be 0.55 μg Se/g LDH, which was lower than what was observed in groundwater (0.65 μg Se/g LDH).…”

“…There have been only a few studies looking at the evaluation of LDH sorbents for selenium removal under dynamic conditions in column tests [29,36,44]. We recently investigated the removal of selenium from natural ground water using granulated LDH (M II = Mg 2+ , M III = Al 3+ , A = CO 2− 3 ) in small-scale column tests [44]. We found that the LDH was effective for removing trace levels of selenium from ground water containing much higher concentrations of sulfate, e.g., [Se] < 2 ppb vs.…”