Despite the benefits of biochar applications to soil, the mechanisms explaining the interaction between biochar and soil properties have not been fully understood. The long-term effects of biochar applications to different soils should also be monitored (Singh et al., 2012). Both qualitative and quantitative assessments of emissions produced during traditional pyrolysis of waste biomass should be carried out to evaluate their effect on occupational health and safety (Verheijen et al., 2010).
Discharge of environmental contaminants from industrial, residential, and commercial sources degrades the surrounding ecosystems. Soil and water media in an ecosystem are frequently subject to contamination by organic and inorganic contaminants mainly due to anthropogenic activities. Technologies are advancing to remediate contaminated soil and water. One of the most important technologies is to reduce the bioavailability of contaminants, and consequently decrease their accumulation and toxicity in plant and animals. Biochar is emerging as an ameliorant to reduce the bioavailability of contaminants in the environment with additional benefits of soil fertilization and mitigation of climate change (Sohi, 2012).
Environmental remediation has been recognized recently as a promising area where biochar can be successfully applied (Cao et al., 2011 and Ahmad et al., 2014). In this review, the effects of pyrolysis conditions, including residence time, feedstock types, temperature and heat transfer rate, on biochar properties, and consequently its efficacy for contaminant remediation are discussed in detail. Special emphasis is given to the mechanistic evidence of the interaction of biochar with soil and water contaminants. Therefore, this review is limited to applying biochar as a green environmental sorbent for the soil and water contaminated with organic/inorganic contaminants.
2. Biochar production and properties
2.1. Biomass pyrolysis
Biomass resources may be limited for the sustainable biochar production. For example, biomass obtained from agricultural crops or plantations as certain types of forests may lead to a decline in soil fertility and an increase in erosion (Cowie et al., 2012). Brick (2010) categorized feedstocks into two groups: (i) primarily produced biomass as a resource of bioenergy and biochar, and (ii) byproducts as waste biomass. Waste biomass has been used extensively for biochar production because of the cost-effectiveness and food security advantages compared to other types of biomass (Brick, 2010).
Biochar can be produced by thermochemical decomposition of biomass at temperatures of 200–900 °C in the presence of little or no oxygen, which is commonly known as pyrolysis (Demirbas and Arin, 2002). Pyrolysis is generally divided into fast, intermediate, and slow depending on the residence time and temperature (Table 1; Mohan et al., 2006). Fast pyrolysis with a very short residence time (<2 s) is often used to produce bio-oil from biomass yielding about 75% bio-oil (Mohan et al., 2006). Slow and intermediate pyrolysis processes with a residence time of few minutes to several hours or even days are generally favored for biochar production (25–35%) (Brown, 2009). Gasification is different with general pyrolysis process. For gasification, the biomass is converting into gases rich in carbon monoxide and hydrogen by reacting the biomass at high temperature (>700 °C) in a controlled oxygen environment and/or steam. The resulting gas mixture is known as synthetic gas or syngas (Mohan et al., 2006).
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