A Review Article Of Biochar Adsorption Of Toxic Pollutants

Biochar is a carbon-rich solid product formed by pyrolysis of bio-organic materials at middle to low temperature 700(degree) under anoxic conditions. The raw materials of biochar are mainly biomass waste (straw, feces, or sludge), which not only benefits waste resource utilization but also effectively reduces environmental disturbance. Biochar has significant adsorption effect for organic pollutants such as antibiotics, phenols, herbicides etc. The organic structure of the biochar is composed of two layers: stacked layers of graphene and aromatic structures which are interspersed with the graphene layer. Soil contamination by heavy metals (HMs, e. g. Pb, Zn, Cd, etc. ) has become more and more serious, which not only makes the soil fertility degradation, crop yield and quality reduction, but also ultimately endangers human health through food chain [1,2].

Applying soil conditioners to remedy heavy metals contaminated soil is a promising way, and has been paid more attention around the world. The common soil conditioners involve lime, phosphate and silicate, and so on. However, these soil conditioners all have issues that the fixation effect of HMs is unstable or new HMs are introduced [3]. Therefore, it is necessary to develop a new soil conditioner which has characteristics of strong remediation ability, high stability and environmental friendliness. Biochar is a solid material produced by biomass thermochemical process under the condition of lack of oxygen (gasification) or oxygen free (pyrolysis), which possesses good pore structure, large specific surface area and a variety of surface oxygen-containing functional groups.

This excellent physicochemical characteristics of biochar is helpful for adsorption and immobilization of HMs in soil, therefore it could be a kind of soil conditioner with great application prospect,Biochar characteristics depend on biomass species and preparation conditions (such as pyrolysis temperature, residence time, pyrolysis atmosphere), which directly affect the remediation effect on soil heavy metals with biochar. Xu and Zhao [6] investigated the adsorption performance of crop straw biochars on Cu(II), Pb(II) and Cd(II) in three variable charge soils from southern China, and found that the peanut straw biochar had better adsorption ability on three metals than canola straw biochar. Heavy metals toxicity i,e a major risk to public health and human –altered environments in all over the world. They mainly act as environmental pollutants and used as a severe threat to animal and human health by their long term perseverance in the environment [27].

Usually most of the heavy metal pollutants are focused on non-essential heavy metals like Pb, Cu, As, Cd, Cr, Hg and essential heavy metals like Cu, Ni and Zn which are toxic and carcinogenic that could cause health problems in humans. Natural constituents of the earth’s crust are Heavy metals Industrialization and technology have led to a rising uses and pollution of heavy metal. In recent years, increasingly more soils are found to be contaminated with organic and inorganic toxins globally due to waste emissions from industrial production, mining activities, waste (i. e. , biosolids and manures) application, wastewater irrigation, and inadequate management of pesticides and chemicals in agricultural production (Bolan et al. 2004; Mench et al. 2010). Biochar for remediation of soils contaminatedwith heavy metalsHeavy metals are not biodegradable, and persist for a long time in contaminated soils. It is expensive and time consuming to remove heavy metals from contaminated soils(Cui and Zhang 2004). Stabilization of heavy metals in situ by adding soil amendments such as lime and compost is commonly employed to reduce the bioavailability of metals and minimize plant uptake (Bolan and Duraisamy 2003; Bolan et al. 2004; Kumpiene 2010; Komárek et al. 2013). Biochar can stabilize heavy metals in the contaminated soils, improve the quality of the contaminated soil (Ippolito et al. 2012) and has a significant reduction in crop uptake of heavy metals.

Therefore, application of biochar can potentially provide a new solution for remediation of the soils contaminated by heavy metals. Stabilization of heavy metals in soils with application of biochar could involve a number of possible mechanisms, as illustrated in Fig. 1 (Lu et al. 2012). Taking Pb2+as an example, the authors proposed various mechanisms for Pb2+sorption by sludge-derived biochar that could include (1) heavy metal exchange with Ca2+, Mg2+, and other cations associated with biochar, attributing to co-precipitation and innersphere complexation with complexed humic matand innersphere complexation with complexed humic matter and mineral oxides of biochar; (2) the surface complexation of heavy metals with different functional groups, and innersphere complexation with the free hydroxyl of mineral oxides and other surface precipitation; and (3) the physical adsorption and surface precipitation that contribute to the stabilization of Pb2+(Lu et al. 2012). In case of acidic contaminated soils, depending on the type of biochars and exchangeable cations (Na, Mg, K, and Ca) present in it could hold the key for the release of some of the these cations during sorption process with the heavy metal, and thus may enrich the stabilization process. Lu et al. (2012) further demonstrated that the heavy metal exchange with Ca2+, Mg2+, and other cations (Na+and K+) associated with sludge-derived biochar was the main mechanism responsible in their study; however, contribution of monovalent (Na+and K+) cations for heavy metal exchange was found to be negligible.

Therefore, it is conceivable that under realistic field situation, sorption mechanisms for metal- contaminated soils by biochar could be dependent on the type of soils and the cations present in both soils and biochar, and thus implications for metal remediation in contaminated soils could vary. The mineral components such as phosphates and carbonates in biochar play an important role in stabilization of heavy metals in soils because these salts can precipitate with heavy metals and reduce their bioavailability (Cao et al. 2009). Cao and Harris (2010) propose that the main mechanism for dairyand innersphere complexation with complexed humic matter and mineral oxides of biochar; (2) the surface complexation of heavy metals with different functional groups, and innersphere complexation with the free hydroxyl of mineral oxides and other surface precipitation; and (3) the physical adsorption and surface precipitation that contribute to the stabilization of Pb2+(Lu et al. 2012).

In case of acidic contaminated soils, depending on the type of biochars and exchangeable cations (Na, Mg, K, and Ca) present in it could hold the key for the release of some of the these cations during sorption process with the heavy metal, and thus may enrich the stabilization process. Lu et al. (2012) further demonstrated that the heavy metal exchange with Ca2+, Mg2+, and other cations (Na+and K+) associated with sludge-derived biochar was the main mechanism responsible in their study; however, contribution of monovalent (Na+and K+) cations for heavy metal exchange was found to be negligible. Therefore, it is conceivable that under realistic field situation, sorption mechanisms for metalcontaminated soils by biochar could be dependent on the type of soils and the cations present in both soils and biochar, and thus implications for metal remediation in contaminated soils could vary.

The mineral components such as phosphates and carbonates in biochar play an important role in stabilization of heavy metals in soils because these salts can precipitate with heavy metals and reduce their bioavailability (Cao et al. 2009). Cao and Harris (2010) propose that the main mechanism for dairy manure biochar to be effective to retain Pb was the precipitation of insoluble Pb phosphates.

Generally, during the manufacture of biochar, water-soluble P, Ca, and Mg increased when heated to 200 °C but decreased at higher temperatures probably due to increased crystallization of Ca–Mg–P, asevidenced by the formation of whitlockite (Ca, Mg)3(PO4)2 when pyrolysis temperature increased to 500 °C, thereby facilitating the precipitation of Pb (Cao and Harris 2010). Alkalinity of biochar can also promote heavy metal precipitation in soils. Chan and Xu (2009) reviewed biochar pH values from a range of feedstocks in the literature and obtained a mean value of pH 8. 1. With the same feedstock material, biochar pH value increases with pyrolysis temperature because of increased ash content in biochar (Wu et al. 2012). Therefore, most biochars are alkaline material and have a liming effect, which contributes to the reduction of the mobility of the heavy metals in contaminated soils (Sheng et al. 2005). However, the adsorption ability of the same type of biochar varies with different types of heavy metals.