Reduction behavior of chromium(VI) with oxalic acid in aqueous solution

The direct Cr(VI) reduction process by oxalic acid was conducted. The existence of Cr(VI) in the reaction medium was measured by software Visual MINTEQ and the concentration of Cr(VI) was measured by ICP-OES. The results showed that the Cr(VI) was efficiently reduced by oxalic acid at high reaction temperature and high dosage of oxalic acid. The reduced product, Cr(III), was easily generated stable complex compounds (Cr(HC2O4)3) with oxalate, which displayed a negative effect on the reduction process. The high reaction temperature and high acidic medium could destroy the stable structure of a complex compound to release oxalate, and facilitate the reduction of Cr(VI). Generally, the results showed in this paper provided a versatile strategy for Cr(VI) reduction and exhibited a bright application future for real wastewater treatment.

Reaction mechanism. During the Cr(VI) reduction process, the predominant Cr(VI) species was HCrO 4 − and the reduction process was reacted following Eq. (1) showed in Fig. 2. The Cr(VI) was reduced to Cr 3+ and H 2 C 2 O 4 was oxidized to CO 2 . The reaction process could be divided into two parts: (I) The formation of esterlike compounds between HCrO 4 − and H 2 C 2 O 4 following Eq. (2). (II) The electron migration between the inner of ester-like compounds following Eq. (3) 21 .
Reduction process. During the reduction process, the effect of dosage of oxalic acid, reaction temperature, dosage of sulfuric acid and reaction time on the reduction efficiency of Cr(VI) were investigated. The residual concentration of Cr(VI) was measured by ICP-OES and the results were shown in Table 1.
The dosage of oxalic acid played an important role during the reduction process as it was the main reaction reagent. Some experiments were conducted to investigate the effect of the dosage of oxalic acid (n(O)/n(Cr)) on the reduction efficiency of Cr(VI) at reaction temperature of 70 °C with 500 rpm. The results shown in Fig. 3b indicating that reduction efficiency was increased with the increase of dosage of oxalic acid. The reduction   www.nature.com/scientificreports/ efficiency was increased from 24.3 to 99.9% as dosage of oxalic acid increased from n(O)/n(Cr) = 1.5 to n(O)/ n(Cr) = 6.0. At the beginning of the reduction process, the reduction efficiency of Cr(VI) was high due to the high concentration of Cr(VI) and oxalic acid and fast reaction rate. Along with the reduction process, the increasing trend of reduction efficiency of Cr(VI) became smooth due to the formation of a soluble Cr(III)-organic products, which formed by Cr 3+ and oxalate (Eq. (4)) 21 . In order to enhance the reduction process of Cr(VI), the high dosage of oxalic acid should be added as there was no enough oxalate to reduce Cr(VI) at a lower dosage of oxalic acid. Thus, the n(O)/n(Cr) = 6.0 was selected as an optimal condition in further experiments. The effect of reaction temperature on the reduction efficiency of Cr(VI) was investigated under the standard conditions: initial concentration of Cr(VI) of 1000 mg/L, n(O)/n(Cr) = 6.0, and stirring rate at 500 rpm. It could be seen from Fig. 3c that the reduction efficiency of Cr(VI) could up to 100% at all reaction temperatures with enough reaction time, and it was easily achieved at a higher reaction temperature in low reaction time, which was partially consistent with a recent study. Higher temperature could increase the activity of atoms and molecules, enforced the reaction intensity, and promoted the reduction process of Cr(VI) 12,13,33 . Meanwhile, high reaction temperature could destroy the stable complex compound and release oxalate, which facilitated the reduction of Cr(VI), thus, the reduction efficiency of Cr(VI) was increased with the increase of reaction temperature. Therefore, the reaction temperature of 70 °C was selected as an optimal condition for further experiments. Figure 3d summarized the effect of stirring rate on the reduction efficiency of Cr(VI) at reaction temperature of 70 °C, n(O)/n(Cr) = 6.0, and it showed that the reduction efficiency of Cr(VI) was all the same as stirring rate ranged from 100 to 500 rpm.
The reduction of Cr(VI) to Cr(III) with oxalic acid could be favoured in the acid condition according to Eq. (1). The Cr(VI) reduction process with oxalic acid was investigated at concentration of H 2 SO 4 ranged from 0 to 300 g/L in this study. Figure 4 displayed that the addition of H 2 SO 4 could facilitate Cr(VI) reduction process. Theoretically, HCrO 4 − was the predominant Cr(VI) species at 0.8 < pH < 6.8, and CrO 4 2− was major species at pH > 6.8 according to the results showed in Fig. 3a, which measured by software Visual MINTEQ 34 , while HCrO 4 − was easier reduced into Cr(III) than CrO 4 2− as HCrO 4 − possessed a higher oxidation potential (E 0 (HCrO 4 − /Cr 3+ ) = 1.35 V, E 0 (CrO 4 2− /Cr 3+ ) = 0.56 V). In the high acidic medium, the complex compound was not stable and released oxalate, which facilitated the reduction of Cr(VI), thus, the reduction efficiency of Cr(VI) was increased with the addition of H 2 SO 4 . Other way, the addition of H 2 SO 4 could improve the reduction efficiency of Cr(VI), the concentration of H 2 SO 4 had no obvious effect on the reduction efficiency at a high dosage of oxalic acid as the oxalate was enough. Integrate.
where v, is the reduction rate of Cr(VI), C, is the concentration of Cr(VI), C 0 , is the initial concentration of Cr(VI), K obs , is the reaction constant. The experimental data were fitted with Eq. (6) and the results shown in Fig. 5a indicating that the data were fitted well with the kinetics model as the coefficient (R 2 ) all closely to 1, in other words, the reduction kinetics behavior of Cr(VI) was followed the pseudo-first-order model equation. The Arrhenius Equation (Eq. (7)) was applied to measure the relationship between K obs and reaction temperature (T) and specific apparent activation energy. The result shown in Fig. 5b showed the simulated Arrhenius equation and the Ea was calculated as 22.49 kJ/mol, which was much larger than the apparent energy calculated for electrochemical reduction (4.74 kJ/mol) 12 . It meant that the reduction process by oxalic acid was harder than electrochemical reduction, while the reduction efficiency was much more efficient ((99.9% for reduction with oxalic acid and 86.45% for electrochemical reduction).
where Ea, is the apparent activation energy, A, is the pre-exponential factor, and R, is the molar gas constant, K, is the reduction rate constant at different reaction temperatures.

Conclusions
In this paper, the direct reduction process of Cr(VI) with oxalic acid was conducted. The following conclusions could be obtained: (1) The Cr(VI) was easily reduced by oxalic acid at high reaction temperature and a high dosage of oxalic acid in acidic medium. Nearly 99.9% of Cr(VI) was reduced at n(oxalic acid)/n(Cr) = 6.0 and reaction  Experimental procedure. All the experiments were carried out in a 250 mL beaker fixed in a thermostatic water bath with a temperature precision of ± 0.1 °C 12,13 . In the batch experiments, a volume of 100 mL solution contained 1000 mg/L Cr(VI) was prepared by dissolving K 2 Cr 2 O 7 in deionized water, then the oxalic acid was added into the solution when the Cr(VI) solution heated to a predetermined temperature. During the experiments, the samples were collected at different intervals (5 min), and analyzed for residual concentration of Cr(VI) in the solution 12,13 . The reduction efficiency (η) of Cr(VI) was calculated as Eq. (8): where C 0 , is the initial concentration of Cr(VI) in the solution, mg/L; Ct, is the concentration of Cr(VI) in the solution at reaction time of t, mg/L. www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.