Phosphomolybdic Acid-Catalyzed Oxidative Degradation of Waste Starch: A New Strategy For Handling The Pulping Wastewater

When old corrugated cardboard (OCC) is returned to the paper mill for repulping and reuse, the starch, which is added to the paper surface as a reinforcement agent, is dissolved into the pulping process water. Most of the OCC pulping wastewater is recycled to save precious water resources; however, during the water recycling process, the accumulation of dissolved starch stimulates microbial reproduction, which causes poor water quality and putrid odor. This problem seriously affects the stability of the papermaking process and product quality. In this study, phosphomolybdic acid (H 3 PMo 12 O 40 , abbreviated as PMo 12 ) was utilized to oxidatively degrade the waste starch present in papermaking wastewater to monosaccharides, realizing the resource utilization of waste starch. The results showed that the optimized yield of total reducing sugar (73.54 wt%) and glycolic acid (11.05 wt%)was achieved at 145 °C with 30 wt% PMo 12 , which is equivalent to 84.59 wt% starch recovered from wastewater. In addition, the regeneration of the reduced PMo 12 was realized applying a potential of 1 V for 2 h. Overall, this study has theoretical signicance and potential application value for resource utilization of waste starch in OCC pulping process and cleaner management of OCC waste paper.

for catalyzing the reactions. These compounds can be redox active in solution by electrochemical oxidation because of its multielectron property, while their anion structure is preserved (Chen et  for 120 min. The glucose yield was more than 50% with 92.3% glucose selectivity, and few byproducts were obtained (Tian et al. 2010). Starch and cellulose are high molecular polymers formed by dehydration of glucose units (Buléon et al. 1998;Goodman 2020). Therefore, POMs can be used to hydrolyze starch to produce reducing sugar (RS) (Mamman et al. 2008; Mua and Jackson 1997) and glycolic acid , having demonstrated excellent electrochemical performance on the degradation of biomass.
Herein, recycled waste starch (WS) in OCC pulping process water was oxidized and degraded using PMo 12 . The in uencing factors, namely, reaction temperature, time, and PMo 12 usage, on the production of RS and glycolic acid from WS were evaluated. Furthermore, the recycling performance of PMo 12 after electrooxidation regeneration was analyzed. The feasibility of turning WS to RS and glycolic acid using PMo 12 was studied. The speci c experimental process is presented in Fig. 1. This work is expected to provide a theoretical guidance and application reference for realizing resource utilization of WS in the OCC pulping process water and developing clean papermaking production.

Materials And Methods
Materials WS from OCC pulping process water was provided by Nine Dragons Paper Co., Ltd (Taicang, China). PMo 12 was supplied by Shanghai Aladdin Chemicals Co., Ltd. (Shanghai, China). Glucose (AR), cesium chloride (CsCl, AR), and diethyl ether (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd.
Curve determination of glucose standard solution Glucose (1 g) was added to a volumetric ask, and deionized (DI) water was added to obtain a volume of 1 L. From this solution, which was labeled as standard solution, 0, 0.2, 0.4, 0.6, 0.8, and 1.0 mL was taken respectively, supplemented with DI water to 1 mL, and 2 mL DNS was added. After reaction in boiling water bath for 2 min, the solution was placed in an ice water bath. Finally, the reaction solution was xed to 15 mL. The absorbance of the mixture was measured using a UV757CRT Model spectrophotometer at 540 nm (Miller 1959;Tian et al. 2010). The DNS reagent reacted with RS to form 3-amino-5-nitrosalicylic acid. The product was brownish red under boiling condition, and the color was proportional to the RS content in a certain concentration range.

Separation of waste starch in OCC pulping wastewater
The wastewater from the multi-disc thickener in the OCC re-pulping was ltrated by Busher funnel with 325 mesh rstly. Then, the ltration was centrifuged at 4500 rpm for 20 min using a high-speed centrifuge. The supernatants were subsequently placed in a dialysis bag with MW-CO 8000-12000(D).Then, a water-soluble organic solution (waste starch and water-soluble organic matter) without the inorganic substances was obtained. The concentration of WS was measured with amylase by HPLC (Lin et al. 2020).
Hydrothermal reaction of WS with PMo 12 WS was easily oxidized and degraded by PMo 12 under hydrothermal conditions. The hydrothermal treatment was conducted in a 250 mL three-port ask reactor equipped with a stirring device and a condenser. A WS solution was prepared and transferred to the reactor, to which a certain amount of PMo 12 was added. Then, the mixture was heated in an oil bath to the desired temperature with a stirring speed of 300 r/min. The effect of hydrothermal temperature was investigated at 140 °C, 145 °C, 150 °C, 155 °C and 160 °C, and the PMo 12 dosage based on WS was investigated from 20 to 80 wt% for 60, 120, 180, and 240 min. Under direct heating, the solution gradually changed from yellow to dark blue. Then, 5 mL of sample was taken every 60 min for standby sampling and diluted to 1 mmol/L, and the reduction degree of PMo 12 was determined simultaneously by absorbance spectrophotometry at 700 nm. To this aim, the standard curve of the reduction degree versus absorbance of PMo 12 was obtained by titrating a PMo 12 solution with a KMnO 4 standard solution (0.01 mol/L).

Determination of total reducing sugar content after the hydrothermal reaction
The PMo 12 present in the mixture after the hydrothermal reaction was removed. After addition of a slight excess CsCl, a precipitate was immediately generated. The upper solution was cleared by centrifugation.
Then, the supernatants were ltered through a lter with a pore size of 0.22 μm, and the ltered liquid contained the total reducing sugar (TRS). Then, a mixture containing 2 mL of DNS reagent and 1 mL of ltrate was heated in a boiling water bath for 2 min, and 12 mL of DI water was added after the mixture was cooled down to room temperature. The color intensity of the mixture was measured in a UV757CRT Model spectrophotometer at a wavelength of 540 nm. The concentration of TRS was calculated according to a standard curve obtained from the concentration of glucose using Equation 1.
where c denotes the concentration of TRS, v is the volume of TRS, and m represents the WS mass.
The concentrations of glycolic acid, 5-hydroxymethylfurfural (5-HMF), levulinic acid, and formic acid were quantitatively determined by an external standard method; then, their quantities in the liquid products of hydrolysis were calculated according to the liquid yields. The concentration of the liquid products in the ltrate was determined by HPLC using an Aminex BioRad-HPX-87H column and a refractive index detector. The mobile phase was 5 mM H 2 SO 4 , the ow rate was 0.6 mL/min, and the column temperature was 55 °C. Two parallel assays were performed for each experiment.

Electrochemical oxidation of PMo 12
The electrolysis cell was composed of a high-density graphite plate with a serpentine groove, a Na on 115 membrane, a silicone gasket, a platinum sheet as cathode, and graphite felt as anode. The bipolar plates of the cell were high-density graphite plates with a serpentine ow channel 2 mm wide, 5 mm deep, and 50 mm long (the total geometry projected area of the channel was 1 cm 2 ), which is schematically illustrated in Fig. 2. The graphite felt was pretreated with concentrated HNO 3  First, the standard curve of this experiment was adjusted to zero with the color reaction solution of DI water as control, and the standard working curve was drawn with the mass fraction of glucose as the abscissa and the absorbance as the ordinate (Fig. 3a). The regression equation was y = 0.0185x + 0.1074 (R 2 = 0.9980), where R 2 is the correlation coe cient and de nes the feasibility of the method and the degree of linear relationship. Generally, R 2 > 0.99 ensures an appropriate limit of error. The DNS reagent was used to determine the TRS content in the experiment.
Effect of reaction temperature on TRS and glycolic acid yield Here, the measured concentration of WS was about 5 g/L by HPLC. Thus, in all experiments, 5 g/L, 100 mL of WS ltrate was applied. 20 wt% of PMo 12 and 120 min of reaction time were applied in this part. As shown in Fig. 3b, the TRS yield increased and then decreased with increasing the reaction temperature.
The highest TRS yield of 62.98 wt% was obtained at 145 °C; the highest yield of glycolic acid was 16.45 wt% at 160 °C. Then, with further increasing the reaction temperature to 160 °C, the yield rapidly decreased to 33.63 wt%, which was 46.60% lower than that at 145 °C. On the contrary, the increase of glycolic acid was 4.92 wt% from 145 °C to 160 °C. Therefore, it can be concluded that the reaction temperature was an important factor that restricts the conversion of starch into RS, and had a great in uence on the TRS yield, meaning that starch could not be fully hydrolyzed to RS at lower temperature. However, as the temperature increased, the oxidation of PMo 12 played a major role, and the oxidative degradation of RS was violent. In acidic solution at a high temperature, RS was the intermediate substance in the process of WS degradation, and was easily degraded in acid medium to produce insoluble humins and other byproducts (Yu et al. 2017). Basically, to prevent ineffective degradation of starch, the optimization reaction temperature is 145 °C, the sum of TRS and glycolic was 74.91 wt%.

Effect of PMo 12 dosage on TRS and glycolic acid yield
Apart from the effects of temperature and time, the dosage of PMo 12 also had a signi cant in uence on the hydrolysis of WS to glucose. The acidity of PMo 12 accelerates the hydrolysis of WS, and the oxidation contributes to the cleavage of chemical bonds of WS, driving the reaction forward. Different TRS yields were obtained by changing the PMo 12 dosage (20, 30, 50, and 80 wt%) at 145 °C for 120 min. For a dosage of 20 wt%, most degradation was observed, and the TRS yield was only 62.98 wt% (Fig. 3c).
Fortunately, the TRS yield increased rapidly to 73.54 wt% and 75.83 wt%, and the glycolic yield was 11.05 wt% to 12.99 wt% when the PMo 12 dosage was 30 to 50 wt%, respectively. However, the TRS yield did not increase signi cantly with further increasing the PMo 12 dosage. Hence, from the perspective of environmental protection and economy, the optimal reaction condition was 30 wt% PMo 12 , which afforded a TRS yield of 73.54 wt% and 11.04 wt% yield of glycolic acid.

Effect of reaction time on TRS and glycolic acid yield
With the optimized reaction temperature and PMo 12 dosage in hands, which were 145 °C and 30 wt%, respectively, the TRS yield was explored at different reaction times. We found that the TRS yield increased rst and then decreased with the reaction time (Fig. 3d). This was because the RS was further degraded under high temperature and acidic conditions. Simultaneously, organic degradation products such as glycolic acid, 5-HMF and formic acid were generated. With increasing the reaction time, the C-C bond cleavage effect of PMo 12 increased(Khenkin and Neumann 2008), which favored the production of levulinic acid and formic acid. Although the TRS yield was only 0.57 wt% higher at 180 min than at 120 min, the reaction time increased by 60 min. Consequently, the TRS yield was reduced. The optimized TRS yield of 73.54 wt% was obtained for 120 min reaction time. In conclusion, the optimal conditions for the catalytic hydrothermal process to achieve a TRS yield of 73.54 wt% were 145 °C reaction temperature, 30 wt% PMo 12 , and 120 min reaction time.
Mechanism of the PMo 12 -catalyzed oxidative degradation of starch As shown in Table 1, the main product of starch degradation using PMo 12 was TRS, and a small number of byproducts (e.g., glycolic acid, 5-HMF, formic acid, and levulinic acid) were detected by HPLC ). Notes: 5 g/L WS, 20 wt% PMo 12 (PMo 12 /WS mass ratio), 120 min reaction time.

PMo 12 regeneration properties
The long-term cycle stability of a catalyst is an important index to evaluate its performance, especially in industrial application. Therefore, the recovery rate of PMo 12 after three cycles and its in uence on the TRS yield were investigated. The absorbance of PMo 12 at 700 nm was used to determine the reduction degree because both variables have a linear relationship (Fig. 5a). The absorbance curve of PMo 12 in a wavelength range from 400 to 900 nm under different hydrothermal reaction times is shown in Fig. 5b. Upon increasing the reaction time from 60 min to 240 min, the reduction degree of PMo 12 also increased, which indicates that WS was degraded. The reduction degree of molybdenum blue (reductive PMo 12 ) decreased gradually in the process of electrooxidation, being gradually oxidized and converted to oxidative PMo 12 (Fig. 5c) recycling performance was still maintained.
The color change of PMo 12 solution during the redox process is illustrated in Fig. 6. Under the hydrothermal reaction, the WS-PMo 12 mixture changed from yellow to dark blue ( Fig. 6a and Fig. 6b . During electrolysis, Mo 5+ in molybdenum blue was oxidized to Mo 6+ at the anode. Simultaneously, the molybdenum blue was converted to oxidative PMo 12 . The color of the solution turned back to yellow, and WS was oxidized and degraded (Fig. 6c). A H + from WS was transferred to the cathode, generating hydrogen.

Conclusions
A large amount of dissolved WS pollutants are accumulated in pulping wastewater during the process of repulping and reuse of OCC waste paper. To solve this problem, we investigated the degradation of WS by the green catalytic oxidant PMo 12 , and the mechanism of the reaction was explored. WS was effectively oxidized and degraded to high value-added RS. During the hydrothermal reaction, at 145 °C, a PMo 12 dosage of 30 wt%, and a reaction time of 120 min, the TRS yield from WS reached 73.54 wt% and glycolic acid (11.05 wt%), which was equivalent to 84.59 wt% of starch recovered from OCC pulping wastewater. Molybdenum blue can be oxidized to oxidative PMo 12 by electrolysis, realizing the recycle and reuse of PMo 12 . The present study on the oxidative degradation of starch by PMo 12 has theoretical signi cance and potential application value for resource utilization of WS in OCC pulping process and cleaner management of OCC waste paper.