The Characteristics Of Different Modified Hydrochar And Their Adsorption Performance For Cu²⁺

With the decreasing of fossil energy and aggravation of environmental problems,the high value utilization of waste biomass is an important way to achieve green and sustainable development.Hydrothermal carbonization is a highly efficient and simple method for waste biomass treatment with low energy consumption.It is not limited by the biomass moisture.High yield of rich chemical functional groups of carbon is one of advantages of hydrothermal carbonization.In this paper,ZnCl₂,Fe Cl₃and SnCl₄were used as catalysts to prepare hydrochar by one-step hydrothermal carbonization using Arundo donax Linn.The solid product-hydrochar was characterized in terms of structure and functional properties.The liquid product was analyzed in components.To further understand the feasible application and potential environmental impact of hydrochar,the release and components of dissolved organic matter（DOM）from hydrochar were studied.Finally,the adsorption experiment was conducted to research the adsorption performance of hydrochars modified with different metal chlorides on Cu²⁺in aqueous solution.And the changes of Cu²⁺adsorption under DOM were analyzed.The results would provide an important theoretical basis and technical guidance for the functional modification of hydrochar and its environmental effects.The results are as follows.（1）Metal chlorides-modified hydrochars（reaction solvents was ZnCl₂,Fe Cl₃ and SnCl₄）were prepared under hydrothermal conditions at 220℃ using Arundo donax Linn and the reaction solvent of control was water.The obtained solid material-hydrochar was characterized using Fourier transform infrared spectroscopy（FT-IR）,field emission scanning electron microscopy（FESEM）and specific surface area（BET）.The hydrothermal liquor was analyzed using total organic carbon analyzer（TOC-VCPH）,ultraviolet-visible spectroscopy（UV-vis）and three-dimensional fluorescence spectroscopy（EEMs）.The experimental results showed that the higher valent of metal,the less oxygen-containing functional groups on the surface of hydrochar,and the more pore structure and larger specific surface area.The BET specific surface area of the hydrochar catalyzed by SnCl₄was 148.64 m²·g^-1（32 times higher than control）.Moreover,with the metal valent increased,the p H value of the liquor decreased from4.0 to 0.7,and the DOC concentration increased from 6.22 g·L^-1 to 15.23 g·L^-1.Components in liquor changed with the varied of metal chloride,mainly the relative content of humic acid-like decreased and the protein-like content increased.（2）The release of DOM from different modified hydrochar was determined by TOC-VCPH,UV-vis,and EEMs.The results showed that the total amount of DOM released from hydrochar showed a rising trend and then stable.And the amount of DOM was increased with the higher valent of metal(78.47、80.38 and 83.23 mg·L^-1 by ZnCl₂,Fe Cl₃ and SnCl₄ respectively,as the control was 59.65 mg·L^-1).The SUVA₂₅₄ values indicated that the hydrochar DOM was a hydrophilic material with low aromaticity.The region integration of EEMs results identified that the fluorescent components of DOM was mainly of protein-like substances,fulvic acid-like substances,microbial metabolites-like and humic acid-like substances.The dominant of hydrochar DOM catalyzed by ZnCl₂and Fe Cl₃ was humic acid-like substances similarly to control,while the dominant component of hydrochar DOM catalyzed by SnCl₄ was protein-like substances.（3）The results of Cu²⁺adsorption in aqueous solutions to different metal chloride modified hydrochars showed that all of them were close to 90%of the maximum adsorption capacity after 10 min and reached adsorption equilibrium at 60 min.While their adsorption processes were in accordance with the pseudo-second-order kinetic model（R²>0.99）.By analyzing of the adsorption isotherms,all modified hydrochar fit the Langmuir model well（R²>0.98）.The higher the metal valent was,the greater the adsorption capacity of Cu²⁺of hydrochar.The maximum adsorption capacity of hydrochar by SnCl₄-catalyzed was 1.24,1.06 and 1.22 times higher than that of the control at 15℃,25℃ and 35℃,respectively.The adsorption process is a spontaneous exothermic process,and the low temperature is favorable for the adsorption reaction.The results of Cu²⁺adsorption on hydrochar under DOM showed that the equilibrium time of Cu²⁺adsorption on hydrochar was constant,and all of them also conformed to the pseudo-second-order kinetic model（R²>0.99）.However,the presence of DOM caused a partial heterogeneous reaction in the Cu²⁺adsorption of hydrochar,which was calculated by fitting the Langmuir isotherm equation.Therefore,the adsorption of Cu²⁺by hydrochar was significantly reduced by Langmuir isothermal equation,and the Cu²⁺adsorption of hydrochar catalyzed by ZnCl₂,Fe Cl₃,and SnCl₄ was reduced by 21.01%,37.38%,40.84%and the control was reduced by 13.13%at 15℃.In summary,addition of different metal chlorides for hydrothermal carbonization could improve the surface structure and pore characteristics of hydrochar.Meanwhile,the high valent metal could change the composition and structure of hydrochar-derived DOM,which makes the released DOM more stable.Therefore,high-valent metal chlorides could be used as an efficient and stable hydrochar catalyst.In addition,DOM derived from hydrochar modified with metal chlorides significantly affected the adsorption behavior of hydrochar and reduced the adsorption of Cu²⁺.Therefore,the application of hydrochar in the environment should consider the potential environmental effects of hydrochar-derived DOM.