CHAPTER 4: PYROLYSIS TEMPERATURE EFFECTS ON YIELD, PHYSICOCHEMICAL
4.4. Discussion
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higher pyrolysis temperature counterparts. The increase in C/N ratio with increase in pyrolysis temperature indicated that the biochar became more recalcitrant. As pyrolysis temperature increases, already existing high C in the form of lignin, cellulose and hemicellulose, is converted into more stable, aromatic and recalcitrant form while on the other hand, low and volatile N is being lost attributed to thermal degradation of biomass (Cantrell et al., 2012) hence increasing C:N ratio. Increase in surface areas and porosity with increase in pyrolysis temperature could be explained by escape of volatile compounds from the feedstock. Increasing pyrolysis temperature enhances the intensity and strength of volatilization of volatile components, leading to improvement in the porosity of the carbonized biomass (Ahmad et al., 2012;
Downie et al., 2011). Kim et al. (2013) found that increase in pyrolysis temperature, greatly increased pH and surface area. An increase in surface area and micro- porosity due to pyrolysis, suggests that water holding capacity, nutrient retention and as well as microbial biodiversity and functions could be enhanced (Masto et al., 2013).
The higher fixed C and increase in C/N of pine-bark biochar with pyrolysis temperature indicated that pine-bark biochar is more important for C sequestration as it is more recalcitrant. This could be essential for C sequestration of high activity C (high surface area and CEC) from pine-bark. The significance of pH and CEC patterns may be explained by loss of acid functional group; O-H stretching of carboxylic acid (Enders et al., 2012). The O-H stretching of carboxylic acid at 350°C may have been converted to O-H bending of phenol (tertiary alcohol) as pyrolysis temperature increased to 550°C. Furthermore, P-O-C stretching of aromatic phosphate (1240 – 1190 cm-1) that got diminished at 550°C may be linked with a slight increase in the intensity of aromatic phosphates (995 – 850 cm-1) at 550°C.
Biochar performance as sorbents depends on the chemical composition and surface characteristics, which in turn are influenced by biochar preparation and feedstock (Li et al., 2014). The positive correlation between Cd sorption and biochar ash content pH, surface area and fixed C suggested that these parameters are essential for increasing sorption of this metal. The positive correlation of Cd sorption with ash content could be a result of exchange reactions with carbonates and other minerals
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in the ash (Inyang et al., 2016). This was in agreement with Xu and Chen (2014) who suggested that minerals in the ash dominated the sorption process. Mohan et al (2007) reported that an increase in pH led to an increase in adsorption of metals wood biochar. The change in biochar pH could have influenced the surface functional groups of the biochar. From the results on Table 4.7, the O-H group of carboxylic acid and the C=O of aldehyde were lost by increasing pyrolysis temperature to 550 and 650oC. Li et al. (2017) reported that for biochar with low CEC, complexation with oxygenated functional groups is an important sorption mechanism for Cd. The contribution of the surface functional groups could have been enhanced by increase in surface area. A review of literature by Li et al. (2017) concluded that surface area was among the most important factors affecting metal sorption on biochar. This view is in agreement with findings from this study which showed that Cd sorption and surface area of pine-bark biochar increased with pyrolysis temperature.
Angelo et al (2014) indicated that changing the biochar's surface chemistry through oxidation may increase retention of metal ions. In some other studies, it is reported that removal of Cd was least influenced by biochar morphology and specific surface area (Trakal et al., 2014). Based on the results of this investigation, these findings are almost in agreement with those Trakal et al. (2014). Furthermore, several other studies reported that pyrolysis temperature influences sorption capacities of different biochars (Trakal et al., 2014; Xu et al., 2013; Inyang et al., 2012; Kolodynska et al., 2012). The results of this study were contrary to other studies, which reported that increase in CEC significantly enhanced Cd removal efficiency of various biochar types (Forjan et al., 2016; Puga et al.,2016; Qian et al., 2016; Bogusz et al., 2015;
Trakal et al., 2014). The difference could be explained by differences in the characteristics of feedstocks used (pine-bark in this study), which affected the properties of biochar.
The higher Cd sorption in the combinations of pine-bark and latrine waste or sewage sludge biochar, compared to expected levels based on proportions and sorption capacities, indicated synergistic behaviour of the component biochars. In Chapter 3, ash and total P content of latrine waste and sewage sludge biochar types were
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positively correlated with Cd sorption, while increase in pH negatively affected sorption. In this chapter (Chapter 4) ash content, pH and surface area are positively correlated with Cd sorption pine-bark biochar. The positive effects of pH and surface area on Cd sorption on pine-bark biochar, while increase in pH decreased sorption on latrine waste and sewage sludge biochars (no effects of surface area), indicates that the mechanisms of sorption are different. Combining the latrine waste (or sewage sludge) biochar, having higher ash and phosphorus, with pine-bark biochar, with higher surface area, could have benefited from both precipitation of phosphates of carbonates and phosphates of Cd and interactions with surface functional groups (Li et al. 2017). The combination could also have modified the surface functional groups and enhanced sorption. The highest synergistic effect at 50% latrine waste and at 75% sewage sludge biochar indicates that these biochar types need to be mixed with pine-bark biochar at these rates for the best removal of Cd from solution.