Oxalic Acid Modification Improves the Removal Effect of Cr(VI) by Biochar through Three Important Means: Adsorption, Reduction and Mineralization

. Oxalic acid, one of the common organic acids secreted by microorganisms, was used to modify the biochar of sludge (SB) and rice husk (RB), and the adsorption function of biochar was combined with the reductive property of oxalic acid to repair Cr(VI). The biochar materials were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform attenuated total reflection infrared spectroscopy (ATR-IR), and the mechanism of chromium hexavalent removal was studied. Adsorption experiments showed that the removal rate of Cr(VI) with the modified SB and RB was 1.68 and 2.19 mg/g within 4h, which was 7.4 and 33 times of that before modification. Oxalic acid modification can enhance the repair of Cr(VI) by biochar in three aspects: 1. increases the BET surface area (109.34-174.13%) and decrease the average pore width (86.3-93.7%) of biochar, which increases the adsorption sites. 2. the addition of reducing oxygen-containing functional groups improves biocahr reduction performance. 3. improve reduced and unreduced Cr ion forms stable minerals on the surface of biochar. Therefore, oxalic acid can not only improve the adsorption and reduction of Cr(VI) by biochar, but also further improve the fixation/passivation of Cr ion after reduction and reduce the migration of Cr ion


Introduction
With the rapid development of industry, the discharge and pollution of industrial wastewater has become a major problem for environmental protection.Among them, Cr contained in industrial wastewater has certain potential risks to the ecosystem and human health (Ma et al., 2021).There are two valence states of Cr in the environment: Cr(III) and Cr(VI).Cr(VI) has higher water solubility and fluidity, and is easily absorbed by plants and penetrated into groundwater, causing great harm to human body, animals and plants (Barrera-Díaz et al., 2012; Johan et al., 2020).The toxicity of Cr(III) is less than that of Cr(VI) (Fu et al., 2011), so it is safer to change Cr(VI) to a low valence state in the repair process.
At present, the methods used to treat excessive Cr ions in water can be roughly divided into three categories: physical, chemical and biological.Traditional treatment in water generally adopts chemical methods (Femina Carolin et al., 2017), but most chemical treatment has high operating cost and inconvenient operation, and it is easy to cause secondary pollution due to overdosage (Mohan et al., 2011).Although the biological method is cost-effective and environmental friendly, the microorganism is vulnerable to the stress of heavy metals, and it is difficult to play the biological repair function (Chen et al., 2020).Physical adsorption method uses substances with high specific surface area or highly developed pore structure as adsorbents to remove heavy metals from water.Physical adsorption method is simple and easy to obtain raw materials, but a range of environmental factors may affect the properties of adsorbent over the long term, resulting in low adsorption capacity.Therefore, it is one of the research hotspots to improve the repair function of adsorbent by means of modification.
Biochar, an excellent adsorbent, is mainly produced by pyrolysis of waste organic biomass.It has the advantages of stable structure, well-developed pores, large specific surface area, abundant surface functional groups and cheap and easy access to raw materials (Tan et al., 2016).Studies have shown that the removal rate of Cr(VI) from apple wood biochar is greater than 99.9% at pH=2 (Liu et al., 2020).There are many modification methods of biochar, such as surface oxidation and reduction, magnetization and biochar functionalization, which can improve the removal efficiency of pollutants (Manik Mian et al., 2018).However, for Cr with variable valence, reduction modification is more conducive to further improve the significance of biochar for Cr repair.
Reducing organic acids have the ability to reduce variable valence heavy metals in the environment.For the treatment of Cr-polluted wastewater and soil, low molecular weight organic acids such as citric acid and oxalic acid have been found to be effective reducing agents (Qin et al., 2020).In this experiment, the physical adsorption of biochar was combined with the chemical reduction ability of oxalic acid, and the modified material was used to repair Cr(VI).We compared and analyzed the Cr(VI) removal effect and related mechanism before and after biochar modification by XRD, ATR and XPS, so as to provide better reference value for the practical application of biochar.

Preparation of modified biochar
5g sludge biochar (SB) and rice husk biochar (RB) were mixed with 100 mL oxalic acid (0.5 mol/L, H2C2O4, 99.8% purity, Macklin) solution, and shook in a shaker at 180 r/min, 28℃ for 12 h.Then the solids after extraction and filtration are fully dried in an oven at 60℃ (48 h).The modified sludge biochar was denoted as HSB and the modified rice husk biochar as HRB.

Adsorption experiment method
The pH of 100 mL potassium dichromate (K2Cr2O7, 99% Purity, Aladdin) solution with a concentration of 50 mg/L was adjusted to 3. 2 g of biochar before and after modification were added to chromium solution, and the four treatments were placed on a magnetic stirrer of 300 r/min for adsorption experiments.Samples were taken at 30, 60, 90, 120, 180 and 240 min, respectively.After adsorption experiment, the four treated solids were centrifuged and collected, which were denoted as SB-Cr, RB-Cr, HSB-Cr and HRB-Cr, respectively.Total chromium in the solution was determined by ICP-OES (iCAP PRO ICP-OES, Germany).Determination of Cr(VI) in solution by ultraviolet spectrophotometer at 540 nm (752N, China).Amount of chromium reduction (Cr(III)) is the difference between total Chromium and Cr(VI).

Material characterization methods
The surface morphology and structure of biochar were determined by scanning electron microscope (SEM) equipped with X-ray energy dispersive spectrometer (EDS) (ZEISS Supra 55, Germany) and automatic specific surface and porosity analyzer (BET) (ASAP 2460, USA).Decaying total reflection infrared spectrometer (ATR-IR) (Nicolet iS5, USA) was used to analyze the functional groups on the surface of biochar before and after modification.The chemical composition of the surface crystal structure of biochar was analyzed by X-ray photoelectron spectroscopy (XPS) (K-Alpha, USA) and X-ray diffrotometry (XRD) (D8ADVANCE, Germany).

Adsorption experiment results
Adsorption experiment showed that (Figure 1), the removal rate HRB (82.62%) > HSB (63.18%) > SB (8.51%) > RB (2.43%), HRB has the largest Cr(VI) removal capacity within 4 h, reaching 2.19 mg/g, which is 33 times higher than RB.The removal amount of Cr(VI) by HSB was also significantly increased from 0.23 mg/g to 1.68 mg/g.From the reduction effect, the reduction rate after modification is significantly increased, the maximum reduction amount of HRB is 0.56 mg/g, and the reduction effect of HSB is better than HRB, reaching 0.65 mg/g.

Material structure analysis
Before oxalic acid modification, the BET surface area of SB and RB was 33.67 m 2 /g and 43.66 m 2 /g, respectively.After modification, the BET surface area of HSB and HRB was 92.30 m 2 /g and 91.40 m 2 /g, which increased significantly (Table 1).The average pore width decreased, which may be caused by the structural collapse of medium and large pores of biochar (Liu et al., 2021).The total pore volume of the two materials changed little before and after modification.The increase of BET surface area provides more adsorption sites for materials, thus increasing the adsorption/binding of Cr(VI) on the surface of biochar.BET surface area of HSB was larger than that of HRB, while removal rate of Cr(VI) of HRB was 1.31 times higher than that of HSB, which indicating that physical characteristics such as BET surface are not the main factors determining the adsorption capacity of biochar for Cr(VI).From the SEM image (Figure 3), the surface of SB has a large number of continuous gullies and deep holes, while RB has a more developed pore structure, in the form of flake.After modification, the surfaces of HSB and HRB become loose and broken to varying degrees, exposing more pores.Therefore, combined with the change of BET surface area, oxalic acid modification can significantly increase the contact area between biochar and pollutants.

Functional group analysis
ATR-IR spectra showed that HSB and HRB had more abundant functional groups than SB and RB (Figure 3).The peak of HSB was more obvious at 1614 cm -1 , and a new peak appeared at 1315 cm -1 and 776 cm -1 .The peak at 1585 cm -1 of HRB shifted to 1614 cm -1 and the peak intensity increased, and a new peak also appeared at 1315 cm -1 .The characteristic peak of 1585 cm -1 and 1614 cm -1 are generated by C=O double bond or C=C vibration on aromatic structure (Liu et al., 2021).1315-1065 cm -1 belongs to C-H in-plane bending vibration and C-O stretching vibration.1000-776 cm -1 is produced by out-of-plane bending vibration of aromatic compounds C-H.These peak changes are likely related to the oxidation-reduction of Cr, especially oxygen-containing functional groups that can promote the adsorption and reduction of Cr(VI) on the surface of biochar to Cr(III) (Ahmad et al., 2014).After adsorption of Cr, the signal of reducing oxygen-containing functional groups on the modified biochar (HSB-Cr and HRB-Cr) decreased significantly, indicating that the reducing functional groups on the modified biochar participated in the reduction of Cr, which is the second main mechanism of Cr(VI) removal by modified biochar.In addition, the vibration peak of HSB-Cr at 536 cm -1 is related to the vibration peak of phosphoric acid group (Zhu et al., 2016), indicating that HSB may form related mineral on the surface after contacting with heavy metals.

Oxidation-reduction performance analysis
The XPS spectrum of biochar after reaction with Cr(VI) is shown in Figure 4

Mineral analysis
SEM images of modified biochar (Figure 5. a and b) showed that crystal particles existed on the surface and pores of the HSB-Cr and HRB-Cr.EDS scanning results showed that these crystals contained a large amount of chromium, which proved that the crystals were chromium-containing minerals.In addition, the XRD results (Figure 5. c and d) showed that there were typical characteristic peaks 2θ=24.61°,36.36°,54.85°and 65.64°on HSB-Cr and HRB-Cr, this peaks may be the typical characteristic peaks of Cr2O3 (Cr mineral standard cards).It is also found that there are chromium phosphate minerals on HSB-Cr, 2θ=20.63°,25.28°3 3.61°and 35.99°.SEM and XRD results further confirmed that biochar adsorbed Cr(Ⅵ) in the solution and reduced it with anionic elements or reductive functional groups on the surface, thus forming a more stable Cr mineral form.In this study, the reaction background pH is 3, so carbonate may not exist stably in a strong acidic environment.Phosphate is more stable than carbonate in acidic environment, so phosphorus provides a new way for biochar passivation and fixation of Cr.Heavy metal mineralization can effectively avoid the desorption of adsorbed heavy metals back to the environment and cause secondary pollution, which is a very effective means to remove heavy metal pollution.In conclusion, oxalic acid modification can not only improve the physical adsorption capacity of biochar, but also increase the mechanism of Cr stable adsorption through functional group optimization and element release, which provides a good reference for the remediation of variable price heavy metal pollution.

Table 1 .
BET surface area and pore size of biochar before and after modification