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. 2019 Jul 2;4(7):11554–11557. doi: 10.1021/acsomega.9b01194

Reducing Hexavalent Chromium to Trivalent Chromium with Zero Chemical Footprint: Borohydride Exchange Resin and a Polymer-Supported Base

John Regan 1,*, Nicholas Dushaj 1, Georgia Stinchfield 1
PMCID: PMC6682018  PMID: 31460261

Abstract

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Aqueous hexavalent chromium, Cr(VI), is rapidly reduced to trivalent chromium, Cr(III), by exposure to (polystyrylmethyl)trimethylammonium borohydride and with Amberlite-supported mild bases in a heterogeneous environment. Post-reaction removal of the insoluble reagents leaves no remediation-based chemical footprint in the source water. Time dependence with stirred and static conditions is discussed.

Introduction

Chromium(VI) is a well-documented health risk1,2 in drinking water, whereas its reduced oxidation state, chromium(III), is a necessary human nutrient used for processing sugars, proteins, and fat. The United States Environmental Protection Agency (EPA) has set a safe standard of 0.1 mg/L or 0.8 μM of all chromium ions.3 Aqueous Cr(VI) is most often found in the form of chromate (CrO42–) or dichromate (Cr2O72–) ions. Under normal environmental pH values, hexavalent chromium is found exclusively as HCrO4– at pH < 6.5 and chromate at pH > 6.5. Cr(VI) in ground water is a byproduct of many industrial processes1,4−7 and can be formed by the oxidation of Cr(III) to Cr(VI) by metal oxides or chlorination.8 Physical methods of Cr(VI) removal used in water treatment sites and some point of use applications depend on adsorption, filtration, or ion exchange technologies.9−11 Alternatively, Cr(VI) elimination strategies rely on the chemical transformation of Cr(VI) to Cr(III). This redox reaction has been accomplished on the interfacial surface of commercial activated carbon adsorbents, Fe(0)12−14 and biochar.15 Soluble reducing reagents, such as l-ascorbic acid,16,17 Fe(II),18 H2S,19 H2O2,20 and sodium borohydride (NaBH4) buffered with sodium borate,21 among others, are also effective.4 However, a serious consequence of removing Cr(VI) under reduction conditions is the introduction of new pollutants to the water source. As an example, the reduction of chromate to Cr(III) with a sodium borate buffered solution of NaBH4 is shown in Scheme 1.21 Inherent in this protocol is the cross-contamination of the water source with sodium borohydride (4 molar excess), boric acid byproducts, and sodium borate buffer (1000 μM) which necessitates an additional remediation step.22

Scheme 1. Reaction of NaBH4, Sodium Borate and Potassium Dichromate at pH 8–10 to Furnished Cr(III) and Boric Acid Species.

Scheme 1

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We report on an environmentally benign approach for Cr(VI) reduction which relies on the use of borohydride exchange resin, A (Scheme 2). Specifically, borohydride is attached to a water insoluble ion exchange resin (Amberlite).23 This reagent has found uses in organic synthesis for the reduction of a variety of functional groups through hydride delivery mechanisms. In other instances, borohydride serves as a single electron donor reducing agent24,25 in the reduction of acridinium cation26 and selenium.27 Furthermore, to avoid contamination of the source water with high concentrations of buffer, we employ insoluble bases to act as mild buffering agents, such as attached to Amberlite (B). A distinct advantage of this heterogeneous reaction (Scheme 2) for the reduction of Cr(VI) to Cr(III) is the simplified post reaction work-ups requiring only filtration of the resin-bound borate reagents (A and C) and bases (B). Afterward, only Cr(III) species would remain in the water.

Scheme 2. Reaction of Polymer-Supported BH4 (A), Polymer-Supported Base (B), and Potassium Dichromate at pH 8–9 Furnishes Reduced Borohydride (C), along with (B), and after Filtration, Only Solutions of Cr(III).

Scheme 2

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Experimental Procedure

Reagents were purchased from Fisher Scientific or Sigma-Aldrich and used without purification. Polymer-supported reagents were obtained from Biotage USA: macroporous borohydride (A) (MP-BH4; #800401; 2.95 mmol/g; 655 μm mean bead size), ISOLUTE Si-Tris-amine (B1; #9495-0010; 1.6 mmol/g), and MP-carbonate (B2; #800267; 2.9 mmol/g). Amberlite IRA-400 chloride form was purchased from Fisher Scientific. Deionized water is used throughout. Spectroscopic measurements of the chromate solution concentration were measured on a Metash Model V-5000 Visible Spectrophotometer at 372 nm. The average of duplicate or triplicate experiments and the range of chromate reductions are reported.

Typical Procedure (Scheme 3) for the Preparation of Amberlite-Supported Base B3

Scheme 3. Reaction of Amberlite (IRA-400 Chloride Form) with N-Cyclohexyl-2-Aminoethanesulfonic Acid in Aqueous NaOH to Furnish B4.

Scheme 3

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To a solution of glycine (1.00 g; 13.3 mmol) in 10 mL water in a 50 mL Erlenmeyer flask was added 1.33 mL of 10 N NaOH. The solution was stirred for 5 min, and 1 g of freshly washed Amberlite (IRA-400 chloride form; 1.4 mequiv/mL) was added. The mixture was stirred 60 min (1″ × 1″ stir bar at 300 rpm), filtered, washed thoroughly with water, and dried at room temperature.

Typical Procedure for the Removal of Cr(VI)

To a solution (20 mL) of aqueous K2Cr2O7 (200 μM) were added MP-BH4 (A) (20 mg) and a polymer-supported base (B) (20 mg). The mixture is stirred for 120 min, or earlier times for kinetic analysis, (1″ × 1″ stir bar at 300 rpm), and an aliquot was removed for spectroscopic analysis. The percent Cr(VI) reduction was calculated in eq 1.

graphic file with name ao-2019-01194a_m001.jpg 1

where λmax[stock] is the UV absorption at 372 nm of the stock solution of Cr(VI). The λmax[sample] is the UV absorption at 372 nm of the sample treated with (A).

Results and Discussion

Sodium borohydride can be utilized as a one electron-reducing agent for the removal of Cr(VI) from water sources.21 However, its use, along with high concentrations sodium borate buffer (1000 μM), presents a challenge for complete water remediation22 by introducing new chemical pollutants. To exploit the reducing ability of borohydride, we adopted a water-insoluble borohydride variant in the form of polymer-supported borohydride (A) which is removed upon completion of water remediation, as in C. To understand the molar requirements needed for a resin-bound one electron-reducing agent to reduce Cr(VI) to Cr(III), we varied the molar ratio of A and Cr(VI). Table 1 shows the results of reducing a 200 μM solution of K2Cr2O7. The results indicate that 10 molar equivalents of A is necessary to completely reduce Cr(VI) levels within 120 min. Fewer molar equivalents of A require longer reaction times (data not shown). The need for excess reagent may reflect chromate’s slow accessibility to some borohydride reagent within the porous resin.

Table 1. Reduction of a 200 μM Solution of K2Cr2O7 in 1000 μM Sodium Borate with Varying Molar Amounts of MP-BH4 (A) after 120 min in a Stirring Environment.

Cr(VI) volume (mL) Cr(VI) μmol MP-BH4 (A) quantity (mg) MP-BH4 (A) μmol molar ratio A:Cr(VI) reduction of Cr(VI) (%)
10 2.0 20 60.0 30:1 100
20 4.0 20 60.0 15:1 100
30 6.0 20 60.0 10:1 97 ± 3
40 8.0 20 60.0 7.5:1 79 ± 6
50 10.0 20 60.0 6:1 75 ± 6
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To determine the minimum amount of buffer needed to slow the decomposition of borohydride to boric acid,28 the concentration of sodium borate was varied. The results in Table 2 indicate that sodium borate is effective at low concentrations—94% reduction of Cr(VI) at 25 μM. However, in the absence of buffer (pH 6.3), the competitive reaction of hydrolysis of borohydride to boric acid occurs faster than chromate reduction.

Table 2. Reduction of a 200 μM Solution of K2Cr2O7 (20 mL) in Varying Concentrations of Sodium Borate with 15 Molar Equivalents of A.

sodium borate conc. (μM) reduction of Cr(VI) (%)
1000 100
100 100
50 97 ± 2
25 94 ± 3
0 25 ± 11
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With these results in hand, we sought to further reduce the remediation footprint by using commercial insoluble mild organic bases (B1–B2) or attaching mild bases to Amberlite to form insoluble reagents B3–B5. Scheme 3 shows the reaction of Amberlite (IRA-400 chloride form) with N-cyclohexyl-2-aminoethanesulfonic acid (CHES) in the aqueous base to form B4.

The physical properties of the bases B1–B5 are shown in Table 3.

Table 3. Physical Properties of B1–B5a.

graphic file with name ao-2019-01194a_0005.jpg

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a

The capacity is calculated from titration values using 0.1 M HCl and phenolphthalein as an indicator. The pH value is taken with 20 mg of B1–B5 in 10 mL distilled water under static conditions.

The results from Table 4 using insoluble bases B1–B5 to aid in pH maintenance indicate that B2–B5 furnish suitable environments for the removal Cr(VI) to occur, with reduction values of 82–99%. Reports on the complexation of chromate with tertiary alkyl amines have been reported,29,30 which in the case of B1 may be responsible for the decrease in Cr(VI) reactivity with the borohydride reagent. With these bases, a range of pH values (7.55–8.96) were effective in controlling the rate of decomposition of borohydride to boric acid.28 After removal of all insoluble material, the source water, containing Cr(III), has a pH value of 8.7–8.9 compared to a pH of 6.3 for the initial 200 μM Cr(VI) solution.

Table 4. Reduction of a 200 μM Solution of K2Cr2O7 (20 mL) with 20 mg of MP-BH4 (A) and 20 mg Bases B1–B5 (30–60 μM)a.

base pH (with reagents) reduction of Cr(VI) (%) pH (after filtration)
B1 8.67 45 ± 1 8.79
B2 8.85 82 ± 1 8.87
B3 8.95 89 ± 2 8.73
B4 8.94 99 ± 1 8.88
B5 8.77 99 ± 1 8.92
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a

The pH values indicate basicity after completion of the reaction and after filtration.

To establish the potential reusability of the bases attached to the insoluble beads, a series of Cr(VI) reduction reactions were undertaken, wherein B4 was confined to a tea bag. After each reaction was complete (30 min), the bag was removed and immersed into a new Cr(VI) solution with MP-BH4. The results (data not shown) show that over the course of three reactions, the percent reduction of Cr(VI) varied by only 9%. Thus, the base-containing beads can be reused in additional remediation reactions.

To understand the contribution of the physical environment to the success of the heterogeneous reaction of chromate with MP-BH4 (A) and B4, the reaction was evaluated under different mixing conditions. Figure 1 shows the rate of Cr(VI) reduction in a stirring versus a static situation. When stirred at 300 rpm, a 95% reduction occurs within 30 min. However, at static conditions, a 9% reduction is observed. An initial rate of Cr(VI) removal in a stirred reaction of 140 μM/10 min. was calculated while the initial rate for static was 8 μM/10 min. The fast initial rate is likely due to a more efficient diffusion of chromate into the porous cavities of A that is required for contact with the borohydride species.

Figure 1.

Figure 1

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Removal of Aqueous Cr(VI) with MP-BH4 and B4.

Conclusions

Synthetic water, containing very high levels of Cr(VI), can be effectively and quickly remediated with the use of two polymer-supported reagents. The reagents attached to the insoluble beads, borohydride and a mild base, reduce Cr(VI) to Cr(III). The water, after filtration of the reagents, will contain only Cr(III) ions and any purification of remediation-based chemicals is not required.

Acknowledgments

The authors thank the School of Science at Manhattan College for financial support.

The authors declare no competing financial interest.

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