Phosphoric acid, with the chemical formula H3PO4, is a polyprotic acid that exhibits distinct chemical characteristics. It is a colorless, odorless, and viscous liquid, soluble in water and capable of forming multiple series of salts. Phosphoric acid's acidic strength depends on its concentration; diluted solutions are weakly acidic, while concentrated ones are more strongly acidic. It reacts vigorously with bases, metals, and oxides, releasing heat and forming corresponding phosphates. Additionally, phosphoric acid can undergo esterification reactions with alcohols and dehydration reactions with carbohydrates. These properties contribute to phosphoric acid's widespread use in various industries, including fertilizers, food additives, and detergents.
Research on phosphoric acid provides a more environmentally friendly solution for treating landfill leachate
With the development of the economy and the increasing urban population, the production of solid waste continues to increase. Landfill method is a solid waste disposal technology commonly used at home and abroad. However, during the landfill process, a large amount of leachate will be produced. Leachate refers to the high-concentration sewage filtered out during the landfill process due to the erosion of fermentation and precipitation, and the soaking of surface water and groundwater. If these leachates are directly discharged without proper treatment, they will pose a threat to the ecological environment and human health. Since there are some organic substances that are difficult to degrade in the leachate, traditional biological treatment methods are not suitable for the treatment of this type of organic substances. Therefore, physicochemical technologies such as ion exchange, membrane filtration and adsorption methods have shown advantages in leachate treatment. In recent years, activated carbon adsorption method has attracted the attention of many researchers. Moreover, this kind of activated carbon prepared from agricultural waste biomass to treat wastewater is conducive to the development of sustainable chemistry and environmental protection. To achieve efficient treatment of leachate with activated carbon, it is very important to understand the treatment mechanism of leachate. However, the mechanism of activated carbon treating leachate is still unclear. In particular, the impact of the structure of activated carbon on leachate treatment requires further study.
Phosphoric acid produced by BLOOM Tech provided product support for this study
Phosphoric acid as an activation agent for the preparation of activated carbon has the advantages of minimal environmental impacts and easy recovery. Herein, the activated biochars prepared from rice husk by phosphoric acid activation were successfully used for the treatment of landfill leachate. The consequent pretreatments by NaOH and HCl removed the silica and ash from biochar thoroughly, which led to cracks and pore opening. Moreover, the phosphoric acid as activation regent could prepare activated carbon with abundant phosphorus groups on the surface. The present study focused on analyzing the performance of variously treated carbon samples on the landfill leachate treatment, revealing the possible roles of phosphorus species formed in activated biochar in the leachate treatment. The applicability of kinetic models and adsorption isotherms for leachate treatment on phosphoric acid activated biochar (H3PO4-activated biochar) were also discussed. It provided important insights to understand the efficient treatment mechanism of wastewater and prepare high-performance activated biochar using agriculture waste residues. Additionally, the utilization of agriculture waste residues for the preparation of high-performance carbon material in the treatment of landfill leachate adhered to the concept of sustainable green chemistry.
Luo Yiping, a postdoctoral fellow at the Chengdu Institute of Biology, Chinese Academy of Sciences (co-supervisor: researcher Liu Xiaofeng), conducted research on the treatment of landfill leachate with activated carbon prepared from rice husk residue using phosphoric acid as an activator. By evaluating the color, pollutant removal rate, COD removal rate and NH4+-N removal rate of the leachate, the performance of carbon samples prepared by different treatment methods in leachate treatment was investigated. It was found that activated carbon prepared by phosphoric acid activation was more effective in treating landfill leachate than carbon samples prepared without phosphoric acid activation. When the mass ratio of phosphoric acid to carbon is greater than 2, the prepared activated carbon shows better performance in the leachate treatment process, namely chroma (100%), pollutants (>90%), COD (~80%) and NH4+-N (100%) has a higher removal rate. Kinetic studies have shown that the process of treating leachate with activated carbon prepared by phosphoric acid activation conforms to the pseudo-second-order kinetics and Langmuir adsorption model. The adsorption process is first controlled by external particle diffusion and then by internal particle diffusion. In the process of activated carbon treatment of leachate, the main organic compounds adsorbed are humic acid substances, and the role of phosphorus species formed in activated carbon is emphasized. The phosphorus species formed in the activated carbon can effectively adjust and control the characteristics and structure of the activated carbon, so that the microporous structure formed in the activated carbon is conducive to the adsorption of humic acid substances. In addition to the pore characteristics of activated carbon, the hydrogen bonding and p-p interactions between the phosphorus species formed in the activated carbon and the humic acid species were found to be important factors in the effective treatment of leachate. The above research results clarify the mechanism of activated carbon prepared by phosphoric acid activation for efficient treatment of landfill leachate, hoping to provide clues for efficient treatment of wastewater in industry.
Materials
Phosphoric acid offered by Shaanxi BLOOM Tech Co., Ltd. Biochar was obtained from Chongqing Jinhuan Biomass Energy Co. Ltd in China, where it was produced after the conversion of rice husk to bio-oil at 400 °C in a fluidized bed reactor. NaOH (AR), HCl (36%–38%) and H3PO4 (85%) were purchased from Chengdu Kelong Chemical Regent Factory in China without further purification. Landfill leachate was obtained from the municipal solid waste landfill site in Luodai, Chengdu, Sichuan Province, China. Before use, it was filtered by a sodium membrane.
Conclusion
The present work studied the performance of phosphoric acid-activated biochars from rice husk for the treatment of landfill leachate, in order to bring its quality to discharge standards. The adsorption of pollutants followed the Langmuir isotherm model and the pseudo-second order kinetics models. The phosphorus species of activated biochar could not only adjust and control the textural properties of carbon, and also interact with the functional groups of humic acid in the leachate via hydrogen.