Study On The Effect Of Preparation Temperature On The Properties Of Pine Bark Hydrochar/biochar And The Cadmium Adsorption Effect Onto Urea Modified Biochar

As a carbon-rich substance obtained by thermochemical methods under oxygen-limited or hypoxic conditions,biochar is a promising environmental remediation material due to its wide variety of functional groups,stable structure,large specific surface area,and high carbon content.In addition,biochar also has broad prospects in the fields of energy recovery,reduction of greenhouse gas emissions and catalysis.The forestry waste generated from log processing is used as the biomass raw material for preparing biochar,which has the advantages of huge quantity,stable source,and strong renewable ability;and the thermochemical conversion method,temperature conditions and modification of the preparation of biochar;and the thermochemical conversion methods,temperature conditions and modification methods for preparing biochar have an extremely important impact on the physical and chemical properties and adsorption performance of biochar.Exploring the appropriate biochar preparation process has far-reaching significance for the rational use and disposal of log processing waste.In this paper,New Zealand radiata pine bark is used as the raw material,and the biochar materials are prepared by hydrothermal carbonization or pyrolysis under different temperature conditions.The main physical and chemical properties of materials are determined by XRD,FTIR,elemental analysis,proximate analysis,Boehm titration and other methods to explore the effect of preparation temperature on the properties of the material,and the effect of temperature on the adsorption performance of cadmium is tested by the batch sorption experiments.At the same time,urea is chosen as the cheap nitrogen source,and various nitrogen-containing functional groups are introduced by the hydrothermal-pyrolysis combined modification method,thereby improving the adsorption capacity of biochar as an adsorbent for cadmium.The specific research results are as follows:（1）With the increase of hydrothermal temperature from 175℃to 265℃,the specific surface area of hydrothermal carbon increased by 1.8 times to 86.97m²/g,and the fixed carbon content increased from 35.51%to 55.41%.But the cation exchange capacity lost 86.21%,dropped to 5.10 cmol（+）/kg,and the yield also dropped from75.18%to 47.83%.H/C,which reflects properties such as carbonization degree and aromaticity,has slightly decreased.O/C which represents stability,polarity and oxygen-containing functional groups,has dropped by more than 50%to 0.27.FTIR showed that the oxygen-containing functional groups of the hydrothermal carbon were damaged to a certain extent,and the Boehm titration method also showed that the total oxygen-containing functional groups decreased slightly.Among them,the loss of carboxyl group was the most significant,from 0.676mmol/g to 0.027mmol/g.The adsorption capacity of hydrothermal carbon decreases with the increase of the preparation temperature.The hydrothermal carbon prepared at 175°C has the best cadmium adsorption performance.However,the displayed adsorption capacity is extremely limited,and the removal rate does not exceed 40%even at a low concentration of0.8mg/L.（2）As the pyrolysis temperature increased from 500°C to 800°C,the H/C of biochar dropped from 0.542 to 0.211,and the O/C ratio dropped from 0.146 to 0.023.The total oxygen-containing functional groups decreased by 23.3%to 2.489 mmol/g.The carboxyl group and lactone group almost completely disappeared,but the cation exchange capacity increased by more than 6 times,from 6.38 cmol（+）/kg to 40.81cmol（+）/kg.The specific surface area increased by nearly 11 times,from 47.04 m²/g to515.54 m²/g.The pH value has also been greatly improved.At the same time,it was also found that as the pyrolysis temperature increased,the cadmium adsorption performance of biochar was significantly improved,and the biochar prepared at 800°C showed the most excellent adsorption performance.At a concentration of 20mg/L,the removal rate reached 85%,and the adsorption capacity was increased by 6.5 times to7.98mg/g,which has a better application prospect.The higher alkalinity in the ash content of biochar may be the reason for the improvement of the adsorption performance of biochar.（3）Both elemental analysis and XPS characterization results show that nitrogen is successfully introduced into biochar after urea modification.The total nitrogen content increased from 0.80%to 4.81%,an increase of 6.01 times.The surface nitrogen content increased from 0.49 to 2.89%,5.90 times improved.The modification method did not significantly affect the yield,Zeta potential and crystal structure of biochar,but it changed the morphology of biochar from broken blocks to micro-nano-scale carbon particle aggregates.In addition,the specific surface area was significantly reduced by nearly 50%to 256.71m²/g,and the average pore size was increased by 3 times to9.11nm.XPS and FTIR test results show that N-H,C-N,C=N and other nitrogen-containing functional groups appear on the surface of the modified material.（4）After modification,the maximum adsorption capacity of biochar increased by1.4 times,from 15.19 mg/g to 20.86 mg/g.In a wide pH range（pH 3～9）,it shows better adsorption effect.The adsorption rate has also increased,reaching 90%of the equilibrium adsorption capacity within 6 hours,while the unmodified biochar needs 12hours to reach the same level.Modified biochar has stronger anti-interference ability against coexisting ions(K⁺,Ca²⁺,Na⁺,Mg²⁺)and humic acid（0～100mg/L）.（5）Through the analysis of the adsorption mechanism,the Elovich kinetic model,the Sips isotherm adsorption model and the adsorption Gibbs free energy change show that the main adsorption process of the biochar to cadmium before and after the modification is chemical adsorption.Ion exchange accounts for a relatively small proportion of the cadmium adsorption of biochar,accounting for 15.52%and 20.12%of the total cadmium adsorption.The characterization results of XPS and XRD show that the precipitation of carbonates is also retained after modification,which plays an extremely important role in the adsorption capacity of biochar before and after modification.In the modified biochar,the complex reaction of nitrogen-containing functional groups with cadmium in the solution was observed by FTIR and XPS.Therefore,the presence of nitrogen-containing functional groups enhanced the adsorption capacity of the modified biochar..