Remediation of heavy metal pollution

Biochar is an ideal candidate for remediation of organic and inorganic pollutants both in contaminated soil and water. This is because of its high specific surface area, micro-porosity and positively and negatively charged surface functional groups (Ahmad et al. 2014). It is used to sorb both organic compounds like pesticides and herbicides and heavy metal contaminants, but it reduces the ability of microbes to breakdown these substances. The sorption of both organic and heavy metals pollutants is influenced by pH, specific surface area, particle size, time of exposure of pollutants and soil moisture. The inorganic ions, or metals can be physically entrapped or chemically sorbed onto the biochar. The adsorptive competition occurs when multi-contaminants are adsorbed onto the biochar. Inyang et al. (2012) investigated the adsorption of Cu2+, Cd2+, Pb2' and Ni2" on the biocliar. They showed that biochar demonstrated better capacity to remove Ni2+ and Cd2+. Soil pH is an important parameter that affects both the surface charge density of the absorbent and the metal ion speciation (Chen et al. 2008). Biochar is alkaline in nature and therefore the increase in soil pH stabilizes metals, except arsenic (Ahmad et al. 2014). The alkalinity also causes some metals to precipitate from the solution on the surface of the biochar, therefore reducing the availability of theses metals to the plants. In addition to this, the heavy metals are less mobile in the soil with neutral pH or above, since the biocliar increases soil pH, which in turn will decrease the mobility of the metals in the soil. Soil pH significantly affects the adsorption of Cu2+ and Zn2+ on the biochar.

Bioavailability and eco-toxicity of heavy metals in biocliar were obtained from sludge of pulp and Pannila and Saroha (2014) studied paper mill effluent. They found that the matrix of biochar heavy metals gets enriched after pyrolysis, but then- bioavailability and eco-toxicity were reduced because of relatively stable fraction. It reduces the mobility of the lead (Pb) due to the formation of insoluble Pb-phosphate as the biocliar derived from manure is rich in phosphate (Chao and Harris

2010). The biochar, which is produced at 700°C, showed the residual form of heavy metals. So, there is no risk of utilization of biocliar as the eco-toxicity of the hearty metals was reduced significantly due to pyrolysis.

Carbon sequestration

Carbon storing capacity of the soil is three times more than that of the carbon existing in the atmosphere. The storing capacity can be further increased if the rate of carbon inputs exceeds the rate of mineral decomposition. Carbon sequestration is the process of storing carbon in soil organic matter and thus removing it from the atmosphere. The biochar potential to sequester the carbon in the soil has received considerable attention in recent years and plays a big role in the climate smart agriculture. Approximately 70-80 per cent of the carbon present in the biomass gets trapped in the structure of the biocliar that is stable in nature compared to the biomass, which on degradation releases the carbon back to the atmosphere. Therefore, it contributes more carbon to the soil compared to the plant residue. So, the production of the biochar and its subsequent addition in soil act as a carbon sink. Biochar generally slows down the decaying and mineralization of carbon cycle to establish a carbon sink. It is the home of beneficial microorganism and the large surface area provides the ability to absorb organic matter, gases and inorganic nutrients.

6.4.1 Biochar carbon persistence in soil

Biochar contains high level of carbon, so it always remains uncertain that for how much tune it will remain persistent in the soil. The persistence level of biocliar carbon in the soil depends upon the characteristics of the feedstock, which in turn depends upon the pyrolysis time and temperature, interaction of the biocliar with the climate condition (precipitation and temperature) and soil environment. A shorter pyrolysis time and high temperature leads to recalcitrant biochar carbon (which remains persistence in soil for longer time). But this is a kind of trade-off between the quality and quantity of the biocliar. as this leads to less production of biocliar compared to per unit feedstock. Soil texture is another criteria controlling the persistence level of the biochar carbon in the soil. Clay particles have more surface area, so there is more interaction between clay particle and the biochar carbon. Therefore, it is more effective in stabilization of the biochar carbon (Coopennan 2016).

6.4.2 Priming effect

The increase in the rate of decomposition of organic matter with the addition of biochar or any other substance is known as the priming effect. This so-called priming effect complicates all the efforts to sequester carbon, as this leads to increase in microbial activity that results in faster rate of microbes decomposition compared to carbon input rates (Coopennan 2016).

Mitigation of climate change

Over the last few decades, the concentration of C02 and other GHGs has increased at a rapid rate, mainly because of industrialization and unsustainable development. This increase in C02 and GHGs generally leads to global warming, which needs to be controlled. So, it has become imperative to mitigate GHG emission by eco-friendly methods and biochar can be one of them. Biochar application to soils may increase carbon sequestration and it leads to increase in recalcitrant carbon pool. But the effect of biochar application on GHG flux is variable among many studies. So, there is uncertainty in carbon sequestration by using biochar in the fields. He et al. (2017) did the meta-analysis of 91 published research papers and did comparison to get the changes in GHG fluxes (i.e., C02, CH4 and N20) in response to application of biochar. They showed that biocliar application significantly increased C02 fluxes by 22.14%, decreased N20 flux by 30.92% and did not affect the CH4 fluxes. Therefore, according to them biochar application may significantly contribute to increase in global wanning potential of total soil GHG fluxes due to high stimulation of C02 fluxes. However, the C02 flux from the soil was suppressed when biochar was added to fertilized soils, indicating that the C02 emission will be suppressed from the agricultural soils where fertilizers (particularly N) are added to the soils. The soil GHG flux varied with source of feedstock, soil texture and the pyrolysis temperature of biochar. To a limiting extent, soil and biocliar pH, biocliar application rate and latitude also influence the soil GHG flux. The biochar affects the soil microbial structure and function, which alters C and N cycling and subsequently changes the soil respiration and N20 flux. Production of the biocliar can reduce the GHG emission by 12 per cent (Woolf et al. 2010). Use of organic matter biocliar in the soil can help in the reduction of C02 (Krishna et al. 2013). It is one of the best methods to sequester the C02 from the atmosphere. Biocliar does not directly sequester the C from the atmosphere, but converts carbon into stable form, hence decreasing the emission of C02 due to decomposition (Kataki et al. 2012). Ibrahim et al. (2017) reported that the emission of greenhouse gases, i.e., carbon (C02) and methane (CH4) was significantly reduced but nitrous oxide (N20) emission first increased and then decreased when amended with biochar.

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