Relationship Between The Chemical Properties And Bioavailability Of Dissolved Organic Matter Released From Hydrochar Of Walnut Shells

One of the ways in which municipal wastewater treatment affects the efficiency of total nitrogen removal due to insufficient carbon sources is to add carbon sources to increase the carbon to nitrogen ratio in the effluent,but liquid carbon sources(e.g.glucose)and solid carbon sources(e.g.polycaprolactone)are expensive.Walnut shells,a typical type of agroforestry waste,are rich in lignocellulosic biomass,and their preparation as hydrochar could be a promising low-cost carbon source for improving microbial nitrogen removal efficiency in wastewater treatment plants.In this study,we investigated the effects of hydrothermal carbonization(HTC)temperature,time,heating rate and solid-liquid ratio on the composition,content and chemical structure of the dissolved organic matter(DOM)released from walnut shell hydrochar.The feasibility of the hydrochar as a potential microbial carbon source was evaluated based on the growth of Bacillus subtilis,and the changes in the physicochemical properties of the hydrochar and its DOM after biological treatment with Bacillus subtilis were compared.The main conclusions obtained were as follows:(1)Confirmed that temperature was the most important factor affecting the properties of the hydrochar.The temperature was increased from 160 ℃ to 280 ℃,the yield of hydrochar decreased from 98.92% to 47.11% and the p H of the process water decreased from 4.91 to 3.23.The degree of dehydration and decarboxylation of hydrocahr increased with temperature and the size of the carbon microspheres produced on the surface increased and became smoother.Optimal conditions for the preparation of hydrocahr suitable for the growth of Bacillus subtilis were obtained.The growth of Bacillus subtilis in aqueous hydrocahr solution was increased by18.32% compared to the control group at an HTC temperature of 160 ℃,a residence time of60 min,a heating rate of 8 ℃/min and a solid-to-liquid ratio of 1:20.(2)The composition of DOM released from walnut shell hydrochar was elucidated.Threedimensional fluorescence spectroscopy-parallel factor analysis(EEM-PARAFAC)showed that the DOM released from hydrochaar from walnut shells was mainly composed of protein,humic acid and xanthic acid.GC-MS analysis showed that phenolic substances such as vanillin and pineal,as well as aliphatic chain alkanes such as tetrapentane and 1,54-dibromo-were the most abundant organic compounds in the DOM.When the temperature reaches 280 ℃,the DOM contains mainly phenolic substances(47.82%)and only 7.05% of long-chain aliphatic hydrocarbons in relative terms.(3)The effect of Bacillus subtilis pre-treatment on DOM released from hydrochar was examined.The Bacillus subtilis treatment increased the proportion of easily degradable substances in the DOM released from the hydrochar prepared at high temperatures(200-280 ℃),thus enhancing its bioavailability,and had a catalytic effect on the conversion of refractory humic substances,such as reducing the toxicity of benzene ring substances and heterocyclic compounds,and Bacillus subtilis could directly utilize the oxygen-containing functional groups on the surface of the hydrocahr,increasing its environmental friendliness.In summary,the walnut shell hydrochar obtained under optimised conditions is a potential source of carbon for wastewater treatment plants,and the enhanced environmental friendliness of the bio-treated hydrochar makes it a green carbon material.Walnut shell hydrochar can be put into industrial trials as a carbon source for sewage plants and its subsequent applications can be developed.