Understanding water retention behavior and mechanism in bio-char

- Water holding capacity of bio-chars correlated well with micropore volume.
- Freezable bound water in bio-chars increased with micropore volume of samples.
- The freezable water and non-freezable water in bio-chars were quantified.
- Phase transition of bound water correlated with pore size distribution of bio-chars.

Bio-char has been applied to soil as an amendment to improve soil water holding capacity due to its porous nature. In this study the behavior of water in bio-char was investigated for better understanding mechanisms of water retention by bio-char. Bio-chars with different structures were prepared by partial gasification of peanut shell (PT) and palm kernel shell (PKS). It was observed that the BET surface area and total pore volume of bio-char increased with gasification conversion. Water holding capacity and water adsorption rate showed a direct correlation with micropore volume of bio-char, suggesting that physical structure of bio-char played a key role during interaction with water. These results indicated that partial gasification is a promising method for production of bio-char suitable for soil remediation. Two types of freezable water i.e. freezable free water (FFW) and freezable bound water (FBW) were directly detected in bio-char samples during Differential Scanning Calorimetry (DSC) analysis. The phase transition temperature of freezable bound water correlated well with pore size distribution of bio-chars. The presence of non-freezable (NFW) water in bio-char was also confirmed from the difference between total water content of samples and the sum of FFW and FBW. The amount of FFW showed an indirect correlation with micropore volume of bio-chars, suggesting that FFW is present in macropores. However, an opposite trend was observed for FBW and NFW in bio-chars, indicating that these two types of water were present in micropores. The low-temperature XRD results were used to define the boundary between the freezable and non-freezable water in bio-char samples.