# Fire-Induced Changes in Soil and Implications on Soil Sorption Capacity and Remediation Methods
## 3 Fire-Induced Changes in Soil Properties and the Implications on Soil Sorption Capacity
### Fire-Induced Changes in Soil Textural Properties and Implications on Soil Sorption
[67] has reported the loss of kaolinite and formation of a glassy phase at soil temperatures above 600 \({}^{\circ}\)C. These observations, however, contradict those of [PERSON] and [PERSON] [16] who showed shifts in kaolinite peaks (\(d\)-spacings), but no complete destruction at similar temperatures. [PERSON] [101] also documented decomposition of kaolinite at temperatures between 500-700 \({}^{\circ}\)C after losing lattice water. Fire-induced changes in smecititic and vermiculite clay minerals have been reported by [PERSON] et al. [31] and [PERSON] and [PERSON] [66], and they too associated these to dihydroxylation. [PERSON] and [PERSON] [66] reported transformation of chlorite and vermiculite to illite and complete decomposition of kaolinite in the top 8 cm of soil as a result of fire events whereas [PERSON] and [PERSON] [16] showed evidence of reduction in 2:1 clay mineral in soils after fire events. Silicate clays and metal-(oxyhydr)oxides are among the major sorbents in the soil environment. The chemical substitution of structural components of layer silicate clays which comprise of stacks of aluminum-oxygen and silicon-oxygen sheets by ions of lower valance gives them a predominant negative charge. These soil components therefore make significant contributions towards soil sorption capacity. These contributions, however, vary with mineral type. Studies by [PERSON] et al. [121] indicate that 2:1 clay mineral have a higher CEC than 1:1 type clay mineral and even within the 2:1 type, smectites contribute more to soil CEC than Illite (Table 3). This difference is associated with the presence of an internal and external surface area in 2:1 clays, which increase the total surface area of the clays as well as its surface charge density. Shifts of mineral assemblage from secondary to primary minerals, as well as dihydroxylation of clay minerals observed in fire-affected soils would \begin{table}
\begin{tabular}{c c} \hline
**Soil Mineral** & **Cation Exchange Capacity (Meq/100 g)** \\ \hline Montmorillonite (smectites) & 80–50 \\ Vermiculite & 100–150 \\ Illite & 20–40 \\ Kaolinite & 3–15 \\ Allophane & 5–30 \\ Gibbsite & 0–4 \\ Hematite/goethite & 4–100 \\ Halloysite & 5–50 \\ glauconite & 5–40 \\ Sesquioxides & 0–1 \\ \hline \end{tabular}
\end{table} 
Table 3: Cation exchange capacity of various soil minerals. result in a decrease in its sorption capacity and consequently its ability to immobilize both organic and inorganic compounds. Oxides of iron (Fe) and aluminum, which usually occur as coatings on clay particles, also play a significant role in soil sorption capacity especially under oxidizing conditions [122]. These metal-(oxyhydr)oxides have a variable charge because their surfaces become hydroxylated when exposed to water. They could be anionic (-MO\({}^{-}\)), neutral (-MOH), or cationic (-MOH\({}^{2+}\)) depending on the degree of protonation they undergo, which in itself is determined by the prevailing pH conditions. They have a net positive surface charge at low pH, a net negative surface charge at high pH, and are neutral at circumneutral pH conditions [123]. Metal oxides have very high temperature thresholds, which may not be encountered by the soil during fire events. Figure 1 presents changes in concentrations of SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), and Fe\({}_{2}\)O\({}_{3}\) in soil heated at different temperatures for different lengths of time. No significant changes were observed when soils were heated below 500 \({}^{\circ}\)C, confirming that these oxides are unlikely to be affected by fire events of low intensity. In studies on both contaminated and uncontaminated soils by [PERSON] [44], few changes were observed in the content of major and trace element concentrations between temperatures of 0-700 \({}^{\circ}\)C, except for MgO, CaO, MnO, and K\({}_{2}\)O concentrations, which increased with increase in temperature and duration of heating.