Water Holding Capacity and Absorption Properties of Wood Chars

The application of biomass char as a kind of soil amendment has an important role in soil water holding capacity (WHC), which has a close relationship with its own surface area, total pore volume, and porosity structure. In this research, the WHC and absorption properties of the chars were investigated. Two kinds of wood (poplar and pine) were pyrolyzed at both 450 and 550 degrees C to produce the chars for the experiments. The Boehm titration was used to measure the concentration of the functional groups. The surface area was determined through the Brunauer-Emmett-Teller (BET) method, while the morphological characteristic of the chars was studied by scanning electron microscopy (SEM). Furthermore, the total pore volume, average pore diameter, and porosity structure of the chars were measured by a mercury porosimeter. On the basis of the pore size distribution of the chars, the definition of the Soil Science Society of America was used as the classification standard. The results showed that there was a significant positive correlation between the WHC of the chars and the total pore volume. However, there was no obvious relationship between the surface area and the WHC of the wood chars. The water absorption rate (WAR) of chars was affected by both the total pore volume and the average pore diameter. The classification of the pore size was needed to further explain the differences of the WAR of the chars. The large pores can not only hold the water in it but also act as the passages to the small pores. The relatively small pore volume of mesopores seriously affected the WAR of the chars.