THE ADSORPTION AND ABSORPTION OF WATER BY WOOD AND GRASS CHARS

Even though chars are frequently referred to as hydrophobic, several studies suggest that the presence of these pyrogenic materials in soil increases the soil's water holding capacity. Chars were produced under controlled conditions (temperature and duration) in the laboratory from plant materials (wood and switchgrass) as well as purified biomass components (cellulose and lignin). Water uptake isotherms were measured by vapor sorption for these biochars. The isotherms were used to evaluate water uptake properties of each char, independent of the effects of particle size, state of soil aggregation, or mineral contributions. These isotherms showed that formation conditions of the chars influenced their water uptake capacity and the relative contribution due to absorptive and adsorptive processes. Low temperature chars (250 to 500 C) produced linear isotherms typical of absorption, while high temperature chars (600 to 900 C) produced Type V isotherms similar to those reported for adsorption by activated chars. Comparison of the volume of water uptake to the available pore volume (measured by nitrogen sorption at 77 K) shows that for low temperature chars, the water uptake greatly exceeds the available pore space providing further evidence of an absorption mechanism. In most cases, the water uptake of the chars equaled or exceeded the water uptake of the uncharred parent material.