Study on city tail water treatment in constructed wetland with straw biochar substrate modified by freeze–thaw cycles

The preparation of biochar is one of the important directions to realize the high-value resource utilization of straw. Constructed wetland (CW) has low cost and high efficiency in treating city tail water. In order to improve the purification rate of CW with straw biochar for city tail water, freeze–thaw cycles (FTCs) modification of straw biochar and the application of FTCs-modified biochar in CW were studied. The relatively optimal modification condition of biochar was eight FTCs. The adsorption of CO2 on eight FTCs-modified biochar was increased by more than 20%. Biochar modified by eight FTCs was used as the CW substrate to promote denitrification, and the nitrate nitrogen (NO3− N) mean removal rate of city tail water was exceeded 85%. The eight FTCs-modified biochar reduced the impact on the electrical conductivity of city tail water. The carbon sequestration rate of eight FTCs-modified biochar reached 55.6%. The research results will be helpful to realize the straw carbon sequestration, improve the performance of biochar and the treatment effect of CW on city tail water.