Method for recovering phosphorus based on cyclic hydrothermal carbonization of wetland plants

A method for recovering phosphorus based on cyclic hydrothermal carbonization of wetland plants, comprising the following steps:

• (1) washing, drying, pulverizing and sieving harvested wetland plants;
• (2) adding biomass powder of the wetland plants to a liquid-phase medium for hydrothermal carbonization treatment to obtain a hydrothermal carbonization product;
• (3) performing solid-liquid separation on the carbonization product obtained in step (2) to collect a solid-phase product and a liquid-phase product;
• (4) mixing the carbonization liquid obtained in step (3), as the liquid-phase medium, with the biomass powder of the wetland plants for multi-stage cyclic carbonization, and after the multi-stage cyclic carbonization is completed, conducting solid-liquid separation to obtain cyclic hydrochar and phosphorus-rich carbonization liquid;
• and
• (5) adjusting a pH of the phosphorus-rich carbonization liquid obtained in step (4) to form and recover phosphorus crystal precipitates, wherein
• the wetland plants are one or more selected from Ceratophyllum demersum L., Canna indica L., Acorus calamus L. and Thalia dealbata,
• in step (2), a mass ratio of the biomass to the liquid-phase medium is 1:5-20, a carbonization temperature is 200° C.-240° C., a holding time is 1-3 h, and a stirring speed is 80-120 rpm,
• in step (2), the liquid-phase medium is water or a 0.5-2 mol/L sulfuric acid solution,
• in step (3), the solid-phase product is the hydrochar, and the liquid-phase product is the carbonization liquid,
• in step (4), a number of stages of the cyclic carbonization is 1-6, and
• in step (5), the phosphorus-rich carbonization liquid is adjusted to a pH of 9.5, stirred at the pH at a rotational speed of 100 r/min for 1 h, allowed to stand for 1 h upon completion of reaction, and the precipitates are collected, washed with distilled water 2-3 times, and dried in a constant temperature drying oven at 50° C. for more than 48 h until the precipitates have a constant weight.