An adsorbent includes a porous substrate and a carboxylic acid dimer loaded onto the porous substrate. The carboxylic acid dimer is loaded on the surface or in the plurality of holes of the porous substrate. The average pore size of the porous substrate is not smaller than 2 nm. The carboxylic acid dimer is loaded onto the porous substrate by at least one of the following manners: a) the carboxylic acid dimer is loaded onto the porous substrate through a Si—OH bond; b) the carboxylic acid dimer is loaded onto the porous substrate through the exchange between a carboxyl group and chlorine; c) the carboxylic acid dimer is loaded onto the porous substrate through the exchange between a carboxyl group and a hydroxyl group; and d) the carboxylic acid dimer is loaded onto the porous substrate through the coordination of a carboxyl group and aluminum or silicon.