3.3.1 Effect of contact time
Removal of Cr(VI) by Z-Fe and V-Fe attained 100% in all the time intervals studied (1 to 60 min) in a solution containing 18 mg L−1 Cr(VI), removing 0.90 mg Cr(VI) per gram of Z-Fe and V-Fe. Reduction of Cr(VI) by Fe(II) occurred very fast, corroborating the results of Liu et al. (2010), who also used Fe(II)-modified vermiculite to reduce Cr(VI) from the solution.
After Cr(VI) removal test, the percentage of Fe(II) remained in minerals composition (Z-Fe and V-Fe) and in solution are indicated in figure 2. In figure 2, the amount of Fe(II) released by minerals minus the Fe(II) found in the solution corresponds to the fraction of iron that was oxidized (removed fractions), while the fraction of Fe(II) released from Z-Fe and V-Fe corresponds to the content of Fe(II) from solution and removed fractions. Although this redox reaction occurs under acidic conditions (eq. 1 and 2), the pH adjustment of the batch test solutions was not performed, as it remained around 5, and did not affect the efficiency of the Fe(II) released from the modified minerals in reducing Cr(VI).
Fig. 2 – Percentage of Fe(II) remained in (a) Z-Fe and (b)V-Fe, and in their respective solution and removed (oxidized) fractions, after Cr(VI) removal test, as a function of time (1 to 60 minutes). The % values correspond to the content of Fe(II) in 1 g of mineral and 50 mL of solution with 18 mg L−1 Cr(VI).
Since three moles of Fe(II) are required to reduce one mole of Cr(VI) (eq. 1 and 2), at least, 2.9 mg of Fe(II) were needed to reduce 0.9 mg Cr(VI). Acidic extraction conducted on 1 g of Z-Fe and V-Fe (before and after the batch tests of Cr(VI) removal) revealed the contents of Fe(II) released from these minerals (Fig. 2). Considering that 1 g of Z-Fe and V-Fe has around 15 mg and 22 mg of Fe(II), respectively, the amount of Fe(II) released from Z-Fe and V-Fe (the removed fraction in figure 2) remained higher than the minimum needed (2.9 mg Fe(II), which corresponds to 19.3% for Z-Fe and 13.2% to V-Fe) to reduce Cr(VI) in all the time intervals (Fig. 2). Z-Fe released more Fe(II) (regardless of time) than V-Fe, whose content of released Fe(II) increased with time.
The release of Fe(II) from Z-Fe probably involved the cation exchange with K+ from the potassium dichromate solution used as the source of Cr(VI) in the batch test, since the K+ removal efficiency of this mineral was high and increased with time (Fig. 3).
Fig. 3 – Efficiency (%) and removal (mg g−1) of K+ from K₂Cr₂O₇ solutions by Z-Fe and V-Fe during Cr(VI) removal test as a function of time.
The remove of K+ concentration in the solution with time (Fig. 3) as well as the release of Fe(II) to the solution (Fig. 2) were higher in Z-Fe than in V-Fe, which further indicates an exchange between K+ and Fe(II). Z-Fe adsorbed more K+ than V-Fe because the exchange K+ ↔ Fe2+ in vermiculite basal space is hampered by the difference in the hydrated radius of the ions. The hydrated radius of K + (0.33 nm) is smaller than that of Fe(II) (0.45 nm) (Strawn et al. 2020) and, therefore, cation exchange requires a collapse of the internal site, i.e., a decrease in the (001) basal space from 14 nm to 10 nm, and this change is quite slow (Sparks 2003). However, this hypothesis must be confirmed by the analysis of V-Fe basal reflection, through DRX analysis (item 3.4).
To verify whether the remaining iron fraction of Z-Fe (5 mg) and V-Fe (15 mg) remained in reduced form, a second set of Cr(VI) removal test was performed with samples underwent 60 minutes batch test. The results revealed that the remained iron was still in reduced form, even after one year. The Cr(VI) removal by V-Fe was higher (43.3% and 0.39 mg g−1 of removal efficiency) than Z-Fe (15.8% and 0.14 mg g−1 of removal efficiency) for having more Fe(II) content in its exchangeable sites. Therefore, V-Fe can still be reused to remove low levels of Cr(VI) in the solution (<10 mg L−1).
3.3.2 Effect of initial Cr(VI) concentration
The increase of Cr(VI) concentration in solution leads to a decrease in the efficiency of its removal by Z-Fe and V-Fe. In tests with Z-Fe, efficiency dropped from 100% in solutions with 18 and 46.8 mg L−1 Cr(VI) to 94% in solutions with 95.3 mg L−1. Similarly in V-Fe, which efficiency went from 100% in solutions with 18 and 46.8 mg L−1 down to 89% in solution with 95.3 mg L−1 Cr(VI).
The mass fraction of Cr(VI) removed from solution in respect to the mass of the adsorbents, increased with increasing concentration of the starting solution; for Z-Fe, this proportion increased from 0.9 mg g−1 for tests with solutions of 18 mg L−1 to 4.5 mg g−1 for solutions with 95.3 mg L−1, while for V-Fe, in the same range of concentration, it increased from 0.9 to 4.2 mg g−1 (Fig. 4).
Fig. 4 - Cr(VI) (a) efficiency (%) and (b) removal (mg g−1) from solution by Z-Fe and V-Fe, as a function of initial Cr(VI) concentration (18, 46.8 and 95.3 mg L−1). Error bars indicate two standard deviations.
Considering again, the need for 3 moles of Fe(II) to reduce one mole of Cr(VI), the amount of Fe(II) required to reduce the Cr(VI) present in 50 mL of solutions with an initial concentration of Cr(VI) of 18 mg L−1 is 2.9 mg (corresponding to 19.3% for Z-Fe and 13.2% for V-Fe); while for solutions with 46.8 mg L−1Cr(VI) is 7.5 mg Fe(II) (50% for Z-Fe and 34% to V-Fe), and for 95.3 mg L−1Cr(VI) is 15.3 mg Fe(II) (102% for Z-Fe and 69.5% to V-Fe). Therefore, the decrease in the efficiency of Cr(VI) removal from solutions with 95.3 mg L−1 of Cr(VI) indicates that the amount of Fe(II) released by Z-Fe and V-Fe was not enough to reduce Cr(VI) available in the solution (Fig. 5). The amount of Fe(II) released from Z-Fe and V-Fe to the solution and the removed fraction, which probably was oxidized forming hydroxides precipitates, increased with the increase of Cr(VI) concentration, from 18 to 95.3 mg L−1.
Fig. 5 - Percentage of Fe(II) remained in (a) Z-Fe and (b) V-Fe, and in their respective solution and removed fractions after Cr(VI) removal test, as a function of Cr(VI) concentration (18, 46.8, and 95.3 mg L−1). The % values correspond to the content of Fe(II) in 1 g of minerals and 50 mL of Cr(VI) solutions.
Since the K+ contents also increased with the increase of Cr(VI) concentrations (from potassium dichromate), in solution with 18 mg L−1Cr(VI) and 13.4 mg L−1 K, 100% of K+ was removed by Z-Fe (corresponding to 0.7 mg g−1K+ removal). The K+ removal efficiency of V-Fe in this solution (18 mg L−1Cr(VI)) was 44.7%, corresponding to a removal of 0.3 mg g−1 K+ (Fig. 6). After the batch-test conducted with 95.3 mg L−1 of Cr(VI), the K+ removal efficiency decreased (92.2%), while the K+ removal increased to 3.3 mg g−1 for Z-Fe (Fig. 6). The efficiency of V-Fe was much smaller (34%), corresponding to a removal of 1.2 mg g−1 K+. This result revealed the high efficiency of Z-Fe in removing K+ from solution likely by ion exchange than V-Fe.
Fig. 6 – Potassium (a) efficiency (%) and (b) removal (mg g−1) by Z-Fe and V-Fe, as a function of initial concentration of Cr(VI) (18 and 95.3 mg L−1).
In contrast to the tests performed with lower Cr(VI) concentrations, Fe(II) was not detected in solution with 95.3 mg L−1Cr(VI), and the removed fraction (oxidized iron) increased with Cr(VI) concentration (Fig. 5). The required amount of Fe(II) to fully reduce the Cr(VI) in 50 mL of a solution with 95.3 mg L−1 was 15.3 mg (corresponding to 69.5%). The actual amount of Fe(II) released in the batch tests was around 11.2 mg for Z-Fe (75%) and 9.9 mg for V-Fe (45%). However, after batch tests with 95.3 mg L−1 Cr(VI), the fraction of Fe(II) remaining in Z-Fe and V-Fe was not released to reduce all Cr(VI) in the solution. Probably, the release of Fe(II) from Z-Fe and V-Fe was affected by the low availability of exchangeable cations in the final solutions.
Iron-rich clay minerals can reduce Cr(VI) and, eventually, the reduced Cr(III) is bound by electrostatic interactions on the permanent-charged sites, by covalent binding with hydroxyl groups, and by cation exchange (Brigatti et al. 2000). In order to evaluate if reduced chromium was adsorbed by Z-Fe and V-Fe, the Cr(III) content was determined in adsorbents (after acidic extraction) and solutions (Fig. 7). Therefore, the difference between the Cr(VI) contents in the samples before the batch test and the fraction of Cr remained in the solution was considered the precipitated chromium fraction (Fig. 7).
Fig. 7 - Percentage of Cr adsorbed in (a) Z-Fe and (b)V-Fe, and in their respective solutions and removed fractions after Cr(VI) removal test, as a function of Cr(VI) concentration (18, 46.8, and 95.3 mg L−1). The % values correspond to the content of Cr in 1 g of minerals and 50 mL of Cr(VI) solutions.
An expressive fraction of Cr(III) ions remained in solution (33% - 58% in Z-Fe and 26% - 49% in V-Fe), while only 9–11% of Cr(III) were fixed in the adsorbents (Fig. 7). The fraction of chromium adsorbed on Z-Fe and V-Fe is quite low compared to Cr(III) available in the solution, probably due to its low selectivity for mineral exchange sites under the experimental conditions.
The fraction (%) of Cr(III) remained in solution increased with the initial concentration of Cr(VI) in Z-Fe, while for V-Fe it increased up to 46.8 mg L−1 Cr(VI), and then, decreased by 95.3 mg L−1 Cr(VI). As part of Cr(III) and Fe(II) remained in solution, the precipitated fraction of chromium was probably in form of Cr(III) hydroxides. In both experiments, the removed Cr(III) fraction (probably due to its precipitation) is the larger fraction at the 18 mgL−1 Cr(VI) test and, then, decreased with the increase of the Cr(VI) concentration. However considering it mass value (in milligram), this precipitated fraction increased with the increase of the Cr(VI) concentration (0.5 to 1.2 mg in Z-Fe and 0.6 to 2 mg in V-Fe).
The presence of Cr(VI) in solution after batch-tests was detected only in experiments with a high initial concentration of Cr(VI) (95.3 mg L−1) (Fig. 7). In those experiments, Fe(II) was not detected in solution, due to its oxidation for chromium reduction. Therefore, the absence of Fe(II) (Fig. 5) and the presence of 6% of Cr(VI) in the 95.3 mg L−1 Cr(VI) solution (Fig. 7) indicate that the amount of Fe(II) released by Z-Fe and V-Fe was not sufficient to oxidize all Cr(VI). Hence, the Cr(VI) reduction is highly dependent on the Fe(II) release (Kwak et al. 2018).