A method of manufacturing a biological absorbent using an agricultural waste comprises: (a) drying the agricultural waste and grounding the dried agricultural waste to particles having about 1mm diameter (b) freezing the particles obtained in (a) to obtain brittle particles (c) grounding the brittle particles and zirconium particles together in a grounder so that the brittle particles being grounded by the zirconium particles into micron porous particles (d) screening and separating the micron porous particles from the zirconium particles to obtain an absorbent having a micron porous structure. The diameter of the absorbent is between 2 to 3[mu]m. The pore size of the absorbent is 88.7nm and the pore volume of the absorbent is about 0.002cm3/g. The absorbent is configured to absorb waste gases. The used absorbent may be recycled by heating to a temperature between 110 DEG C to 120 DEG C for 2 to 15 hours. The recycled absorbent may have absorbent ratio of 92% to 103% compared to the original absorbent. Therefore, the recycled absorbent may still have a high absorbent capability.
