Removal of Aqueous Cr(VI) by TiO2 Semiconductor Photoreduction/Precipitation

Interest in semiconductor photocatalysis for wastewater treatment has received much attention lately. It is based on electrons and holes generated on the semiconductor surface when illuminated by light with energy greater than the bandgap energy. These electrons and holes recombine or participate in redox reactions. TiO2 is the commonest photocatalyst due to its favorable chemical properties, high stability and low cost. The TiO2 bandgap energy is 3.2 eV (corresponding to 400 nm UV). The holes are oxidizing and can be utilized in organic compound degradation. Furthermore, inorganics with a reduction potential more positive than the conduction band can consume electrons and recently more attention has focused on these compounds. TiO2-photocatalytic reduction is reported to effectively remove toxins: Hg(II), Ag(I), As(V)/As(III) and Cr(VI). Although studies on Cr(VI) reduction over semiconductor catalysts exist, sparse information exists for co-reactants, except oxygen. Photocatalytic Cr(VI) reduction with Fe(III) was therefore considered in this work.

Because chromium is common in wastewater, there have been many studies to determine the effectiveness of its removal during treatment. The first step is conversion of Cr(VI) to Cr(III) by reductants, followed by Cr(III) removal. Lime softening, alum coagulation, and iron coagulation have been found to be effective. Recently, research has shifted to overcome the conventional approach limitations. Because UV/TiO2 photocatalysis only reduces Cr(VI) to Cr(III) (which is still toxic), photoreduction/coprecipitation was considered for Cr(VI) removal to achieve better results. 

Experiments were conducted in a 200-mL Teflon reactor. The mixture was stirred, pH adjustments were made using dilute HNO3 and NaOH. The irradiation source was a 450 W Xe arc lamp with a UV-VIS band-pass filter. The UV emission is above 290 nm with an intensity of 2.202 mW/cm2. A solution containing 600 μM Cr(VI) and 20 mM formic acid was used. The suspensions were kept unilluminated for 60 minutes to ensure an adsorption-desorption equilibrium. During irradiation, aliquots were withdrawn intermittently. 

Figure 1: Photocatalytic reduction of Cr(VI) in 2 g/L TiO2 suspensions.


Figure 2: TiO2 conduction band levels and metal reduction
potentials with pH.

Cr(VI) reduction profiles are shown in Figure 1. Based on Figure 2, the Cr2O72-/Cr3+ reduction potential shifts 138 mV/pH to more cathodic potentials, whereas the TiO2 conduction band shifts 59 mV/pH; Cr(VI) reduction tendency decreases with increasing pH. The decreasing rate may also be attributed to Cr(OH)3 deposition on the catalyst surface for pH greater than 5.

The Cr(VI) adsorptivity decreased with pH. Adsorption is mainly attributed to surface properties. TiO2 Degussa P25 is a nonporous mixture of anatase and rutile with a 70:30 ratio. Positive charges on TiO2 surface decrease with pH, reaching zero at pHzpc. For pH greater than 6, negative charges on TiO2 surface increase; CrO42- adsorption is reduced. 

Figure 3: Fe(III) influence on Cr(VI) reduction at pH of 2.5 on 0.4 g/L TiO2.

Figure 3 shows Cr(VI) reduction with and without Fe(III) as an additional electron transfer reagent. The higher yield for Cr(VI) photoreduction was due to Fe(III) and Cr(III) hydroxides. It has been reported that ferrous ions from Fe(III) photoreduction promote Hg(II) reduction. 

Without UV, the Fe(II) concentration decreased gently, because it was oxidized by Cr(VI). The Fe(II) concentration increased sharply, when under UV irradiation after 70 minutes. Comparing the Cr(VI) reduction profiles, the rates during the initial 10 minutes were identical. 

In conclusion, the reduction of Cr(VI) by UV/TiO2 photocatalysis has been shown. An acidic medium is favorable for Cr(VI) photoreduction, where a 94% yield for Cr(VI) photoreduction within one hour was obtained. The photoreduction mainly occurs on the TiO2 surface and the addition of Fe(III) improved it. Photoreduction/coprecipitation using Fe(OH)3 for Cr(VI) removal (from 30 ppm to 17 ppb) has been designed.