Study on Preparation and Properties of TiO 2 /Biochar by Phosphoric Acid Modifition

: In this study, phosphoric acid was proposed as an activator to mix and activate the biomass and tetrabutyl titanate, and the parameters were adjusted to obtain the optimum conditions for the preparation of photocatalysts. The specific surface area of the composite photocatalysts prepared under the high ratio (phosphoric acid: peanut shell = 2:1) of phosphoric acid modification was obtained to be 317 m 2 ꞏg -1 , with an average pore size of 32 nm and a large number of hydrophilic groups on the surface of the material to improve hydrophilicity. The results showed that the adsorption and degradation of methylene blue by the one-step modified composite photocatalyst was 102.5 mg/g, and the adsorption-degradation efficiency reached 99.4%, which had excellent water treatment effect and improved the application possibility of the composite photocatalyst in water treatment.


Introduction
Dyes in dye wastewater are characterised by poor biochemical properties, high colour, chemical stability, high oxidation resistance and difficult biodegradation [1] , which are not suitable for biological treatment or physical adsorption treatment. Among the various wastewater treatment methods, advanced oxidation techniques (AOPs) are of wide interest [2,3] , where photocatalytic oxidation is commonly used to load TiO 2 onto the substrate and reduce TiO 2 deficiencies. And recently, it has been shown that acids can activate biochar precursors to achieve good carbon production, and can also promote the crystal growth of TiO 2 [4] . In this study, a mixture of biochar and tetrabutyl titanate was activated with phosphoric acid and a one-step anaerobic preparation calcination was used to prepare the composite photocatalyst, on which the effect of H 3 PO 4 modification on the degradation of methylene blue by TiO 2 /BC composite catalyst was investigated.

Experiments
Materials：The raw materials used for the experiments included peanut shells; tetrabutyl titanate; methylene blue (Methylene Blue); orthophosphoric acid (85 wt%); and homemade deionized water.
Preparation ： The peanut shells were washed with deionised water, dried and sieved with a blower, and sealed for use. The peanut shells were mixed with tetrabutyl titanate mixed with ethanol in a certain proportion, and after thorough mixing, phosphoric acid was added in a certain proportion for secondary mixing, and then left to hydrolyze for 4h at room temperature, dried until dispersion, calcined in a nitrogen atmosphere with a heating rate of 10K/min, and held for 2h at 500°C. The products were fully ground and washed with deionized water, and the products obtained were labelled as BC, TiO 2 /BC-1, TiO 2 /BC-2, TiO 2 /BC-3, TiO 2 /BC-4. Figure 1 shows SEM images of the synthesised materials TiO 2 /BC-1 and TiO 2 /BC-4. The catalysts modified with a low proportion of H 3 PO 4 showed a laminar petal structure and a large number of pore channels, increasing the adsorption surface area, but still retaining the morphological structure of the biomass itself; with the incremental value of the phosphoric acid addition proportion, a large number of pore channels appeared on the surface of the composite catalysts, the surface breakage was further intensified, the surface was rich in hollow and pore structures, the pore channels and hierarchy were obviously incremental, the rich laminar structure greatly The rich layered structure greatly expanded the adsorption specific surface area of the composite catalyst, increasing the loading sites of TiO 2 and effectively expanding the active sites of photocatalytic degradation. Figure 2(a) shows the sorption and desorption curves of the nitrogen gas samples, vertically, the top-down sorption and desorption curves gradually show a tendency to close. The surface area of the modified composite catalyst gradually increases and the functional groups on its surface are cross-linked by the modification, reducing the effect of the functional groups on gas adsorption and desorption. Figure 2(b) shows the pore size distribution of the samples, which shows that the peak pore size of the unmodified biochar and the low concentration H 3 PO 4 modified composite catalysts appeared around 10 nm. The adsorption-desorption curves of the two samples were characterised by capillary coalescence followed by hysteresis loops, which belonged to type IV structures, suggesting the presence of good mesoporous structures in the composites after modification with high proportions of H 3 PO 4 . This change may be due to the cross-linking effect of H 3 PO 4 and H 3 PO 4 as both an acid catalyst to promote bonding and cross-linking of biopolymer fragments via H 3 PO 4 salts or poly H 3 PO 4 salt bridges. The addition or insertion of the H 3 PO 4 group leads to swelling within the precursor matrix, which, upon removal of the acid, leaves the matrix in a swollen state, resulting in a pore structure [5] . The TiO 2 /BC-4 sample was able to achieve a specific surface area of 317 m 2 -g -1 with an average pore size of 32 nm, which is higher than other reports in the literature for similar materials.

Infrared absorption spectroscopy (FT-IR) characterisation
The functional groups on the surface of the material play a decisive role in the adsorption-degradation ability of the photocatalytic material, so we used Fourier transform infrared spectroscopy (FTIR) analysis to determine the functional groups present on the surface of the prepared activated carbon and to understand the mechanism of adsorption of methylene blue by the activated carbon. It can be observed from the Figure 3 (a)that the FTIR spectra of all the modified catalysts showed almost similar trends but with a decrease in peak intensity, as the cross-linking reaction is the main mechanism for the retention of char in the solid phase, and throughout the cross-linking reaction P atoms are able to form H 3 PO 4 salts and poly H 3 PO 4 salt bridges, which cross-link the polyaromatic element fragments into char crystals, and in the presence of phosphorus, the organic matter rich in biomass is successfully converted from natural organic macromolecules This is one of the reasons for the reduced peak intensity after the modification. Specifically from individual samples, the peaks in the 3400 and 1650 cm -1 regions in BC correspond to the O-H stretching and bending vibrations of the hydrogen bonded hydroxyl group (OH) of cellulose and absorbed water [6] , respectively, with a reduced OH content in the H 3 PO 4 modified dehydrated raw modified composite catalyst; the C=C aromatic skeleton vibrations of lignin at 1500 ~ 1600 cm -1 . The peak at 972 cm -1 is the C-H stretching and C-H bending vibration of cellulose [7] O stretching and C-H wobble vibrations of cellulose + , 1089 cm -1 probably due to the electric power of P -O and symmetrical vibrations of P-O-P in the poly H 3 PO 4 chain [8] , H 3 PO 4 modification breaks down the C-P groups to form new P-containing groups, such as P-(OH) 3 , which are also able to increase the hydrophilicity of the composite catalyst.
From the IR spectra of the titanium dioxide and sample TiO 2 /BC-4 in Figure 3(b), it can be seen that a distinct peak appears at 500-700 cm -1 , where the characteristic peak of titanium dioxide is present, indicating that the titanium oxide prepared by the one-step process has most likely been loaded into the biochar. The characteristic peak of methylene blue can be seen in Figure 3(c) at around 1000-1250 cm -1 . The composite catalyst following the catalytic reaction under light conditions showed no significant change compared to the peak of the unreacted photocatalytic material, but the catalytic material under dark conditions for adsorption showed a significant wave at around 1200 cm -1 , indicating that methylene blue had been adsorbed onto the This indicates that methylene blue has been adsorbed onto the catalyst and can be degraded under light conditions, which demonstrates the synergistic effect of adsorptionphotocatalytic degradation of the prepared composite catalyst.

XRD characterisation
The XRD spectra of the composite catalyst  [9] , and the diffraction peaks of the composite catalyst at 2θ =25.74, 37.53, 49.66° coincide with the rutile crystal plane of the standard diffraction spectra (JCPDS 81-0791), so the precursors of H 3 PO 4 modified composites may reduce the temperature of anatase to rutile conversion. Figure4(b) shows the Raman spectra of samples. Based on a review of the literature [10] , the Raman shift wave numbers for anatase are around 399, 515 and 639 cm -1 , but due to the low content of TiO 2 in this sample, only a faint peak at 385 nm can be seen for TiO 2 /BC-4, which may be titanium dioxide already loaded onto biochar. By comparing the peaks by graphing, the overall D peak was higher than the G peak for all three materials, and the overall disordered carbon atomic material. The I D / I G for TiO 2 /BC-1, TiO 2 /BC-3 and TiO 2 /BC-4 were calculated to be 0.81, 0.85 and 0.87, indicating that the degree of graphitization gradually decreased after modification, which may be due to TiO 2 binding to the carbon layer and reducing the degree of graphitization of the biochar. graphitisation of the carbon.   Figure 5 shows the adsorption and degradation performance curves of the five materials on the MB solution of 20 mg-L -1 under light. 5 samples had high adsorption rate in the early stage of adsorption, slightly decreased in the middle stage, and the adsorption rate gradually leveled off in the later stage, which may be due to the large number of free sites on the surface of the active material before adsorption, with the saturation of the surface adsorption, under the action of molecular pressure, the dye molecules passed through the pore size of the material into the internal pores of the molecules, producing secondary adsorption. As the phosphoric acid ratio increased, the functional groups on the surface of the active material increased and the porosity of the active material increased, the high ratio phosphoric acid modified active material reached adsorption equilibrium faster in the first two stages, and this explanation is also consistent with the BET test results. The low proportion of phosphoric acid had less erosive effect on the peanut shells and was not able to produce enough pores and pore channels, resulting in a smaller specific surface area for the adsorption of organic dyes, indicating that trace amounts of phosphoric acid were less effective in modifying the catalysts. The catalysts modified with a high proportion of phosphoric acid (phosphoric acid: peanut shells = 2:1) showed a significant increase in the adsorption efficiency of MB and were able to reach adsorption equilibrium within 30 min, as calculated by the light The adsorption-degradation rate of pollutants in the simulated wastewater was 98.11% and the adsorptiondegradation amount was 102.5 mgꞏg -1 under light conditions.

Kinetics, adsorption isotherm experiments
The model parameters and R 2 values for the Langmuir and Frereundlich isotherms are listed in Table 1. From Table  1 it can be seen that the Langmuir adsorption isotherm R 2 value is 0.9866 and the Freundlich adsorption isotherm R 2 value is 0.7531, the Langmuir model fit has a relatively high R 2 value, therefore it can be concluded that the RhB dye on a homogeneous surface The conclusion that monolayer adsorption was used for the adsorption of RhB dye on homogeneous surfaces. Figure6 shows the adsorption kinetics plotted from the primary and secondary kinetic models. The equilibrium adsorption amount qe obtained from the calculations matches the actual adsorption data, the value of R 2 is larger and close to 1 in the secondary kinetic model, which indicates that the rate of adsorption of RhB by the modified catalyst prefers secondary kinetics, the secondary rate equation can be inferred for the whole adsorption process, indicating a good match with the rate control step being chemisorption, it is not the concentration of the dye that affects the rate of adsorption but probably the chemisorption, including electron sharing or electron exchange between the adsorbent and the adsorbate [10] , suggesting that this material is a photocatalytic material capable of monolayer adsorption on a surface and photocatalytic degradation.

Conclusion
In this study, phosphoric acid was used as the activation modifier to modify the catalyst with peanut shells and tetrabutyl titanate as precursors, and the maximum surface area of the composite catalyst was 317.21 m 2 /g, and the adsorption of methylene blue was 102.5 mg/g. The experimental results showed that phosphoric acid was able to modify both biochar and TiO 2 , significantly improving the catalyst performance and reducing the agglomeration of TiO 2 . The effect of H 3 PO 4 on the photocatalytic performance of TiO 2 /biochar was reflected in the combined effect of changing the specific surface area of the catalyst, its ability to absorb light and its adsorption and activation performance on methylene blue. The composite catalyst showed good adsorption and photocatalytic performance for methylene blue alkaline dyes. The adsorption and degradation of methylene blue by the TiO 2 /BC-4 catalyst reached 99.4%, which was significantly better than that of the unmodified biochar, and the adsorption composite secondary kinetic model. Therefore the composite catalyst prepared by this method is suitable for the adsorption degradation treatment of dye wastewater.