# 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  
Though soil particles have high threshold temperatures (460-980 \({}^{\circ}\)C for clay and 1414 \({}^{\circ}\)C for sand), several authors have reported that wildfires affect soil particles and consequently soil texture [26; 48; 49; 50; 68]. Clay particles begins to experience changes in structure at temperatures of 400 \({}^{\circ}\)C due to hydroxylation and collapse of the clay lattice structure, and can be completely destroyed at 700-800 \({}^{\circ}\)C. This occurs mainly because of the dominance of secondary minerals in the clay fraction of soils. Sand and silt particles on the other hand are more thermally stable because they comprise mainly of quartz, which has a melting point of 2577 \({}^{\circ}\)C as well as a high thermal conductivity, and can only be affected under extreme soil temperatures not normally attained during wildfire events. [PERSON] et al. [68] showed that clayey soils are more susceptible to fire-induced changes compared to sandy and silty soils, and he attributed this to differences in their mineral composition. According to [PERSON] and [PERSON] [66], [PERSON] et al. [67], and [PERSON] et al. [113], soil temperatures above 600 \({}^{\circ}\)C result in a sharp increase in sand content and a decrease in silt and especially clay content in soils. [PERSON] et al. [69] have associated the increase in particle size of soils after fire events with the conversion of CaO to Ca which results in agglomeration, whereas [PERSON] et al. [78], [PERSON] et al. [86] and [PERSON] et al. [114] have implicated swelling of ash particles. Destruction of kaolin during fires results in the production of oxides and hydroxides of Al and Si which are cementing agents capable of aggregating clay particles into sand-sized ones [115] that could also result in the observed increase in coarseness of soils after fire events. Accumulation of sand-sized particles in soils after a fire event is likely to reduce soil sorption capacity. [PERSON] et al. [116] have shown that soil texture influences contaminant sorption in soils by up to 37.1% with finer soil particles playing a major role. This influence has been attributed to the cation exchange capacity (CEC) of the various soil particles which, according to [PERSON] and [PERSON] [117] increases from 10 mmol/kg in sandy soil to 813 mmol/kg in fine clay (Table 1). The role of soil texture in the sorption of inorganic and organic contaminants is well documented. Studies have shown that absorption of Diuron (a urea herbicide) on soil particles for example is three times higher in the <50 \(\upmu\)m fraction of soil compared to the 50-2000 \(\upmu\)m fraction [118]. The concentration of polycyclic aromatic hydrocarbons (PAH) in the <63 \(\upmu\)m fraction of river sediments has also been shown to be higher than in the 1000-2000 \(\upmu\)m fraction, indicating a higher sorption of PAH by fine particles [119]. Studies by [PERSON] et al. [120] have indicated that the electrochemical properties of soils \begin{table}
\begin{tabular}{c c} \hline \hline
**Soil Particle** & **Cation Exchange Capacity Range cmol (+)/kg** \\ \hline Fine clay & 57.4–81.3 \\ Coarse clay & 33.6–74.9 \\ Whole clay & 20.9–110.0 \\ Fine silt & 6.8–41.4 \\ Medium silt & 63–34.5 \\ Coarse silt & 1.2–12.8 \\ Sand & 1.0–15.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Cation exchange capacity of soil particles. are not only contributed by the clay fraction, but silt particles, especially those with diameters between 2 and 20 um, also play a major role as well. Silt fraction of soils contribute up to 17.5% of CEC to the total soil CEC [120]. Increased sand-sized particles in soils after fire events reduces the total soil surface area and surface charge density with a consequent reduction of their CEC. This would affect the overall sorption capacity of the soil and its attenuation of both organic and inorganic contaminants.