The Research of Biochar Adsorption on Soil

Jie Chen a, Youyang Wangb, Junhui Wuc*, Huiping Sid, Kaiyan Line

National Engineering Research Center of Protected Agriculture,

Modern Agricultural Science & Engineering Institute, Tongji University, Shanghai, 201804, China

a1968chenjie@163.com, bvivian_114@126.com, cjunhui_wu@163.com,

dsihuiping@tongji.edu.cn, eky.lin@163.com

Keywords: Biochar Sorption, Soil Fertility, Organic Contaminants, Heavy Metal Pollutions

Abstract. This article discusses biochar adsorption and its effects on soil and discusses the future trends in this area. The large surface area, and many oxygen-containing functional groups of biochar determined by the feedstocks and the condition in pyrolysis affect the capacity of biochar to adsorb fertilizer, water, heavy metals and organic pollution. With enriched porous structures, biachar can increase the porosity and water retention capacity of soils. With the functional groups and the composition, biochar have a high adsorption capacity for fertilizer, heavy metals, organic pollutants. This paper provides an overview on the biochar sorption in fertilizer, heavy metals and organic pollutants in soil and its implication for soil to keep soil fertilizer as a controlled-released carrier and to improve soil environments as landscaping organic mulch, as well as for remediation of contaminated soils. Further research directions are identified to ensure a safe and sustainable use of biochar as a soil amendment.

Introduction

Biochar, namely biomass-derived charcoal, refers to the highly aromatic substance remaining after pyrolysis and carbonification of biomass under complete or partial exclusion of oxygen, which can be produced from a wide range of biomass sources including woody materials, agricultural residues such as corncobs and crop straw, livestock manures, and other waste products. Soil amendment with biochar is evaluated globally as a means to improve soil fertility and to mitigate climate change. When applied to soil, biochar was of great importance in increasing soil carbon storage, improving soil fertility, as well as maintaining the balance of soil ecosystems, and it could act as a kind of soil fertilizer or amendment to increase crop yield and plant growth by supplying and retaining nutrients.

[1,2,3].

A lot experiments have demonstrated that the biochar adsorption of fertilizer, the heavy mental ,organic material and so on play an important roles in retaining nutrient and decreasing pollution , the biochar was used as a soil amender. The key role of Biochar adsorption is keeping fertilizer from leaching loss and avoiding pollution from plants. The researches have reported that the biochar sorption was largely controlled by the physical and chemical properties, determined by the feedstock of the preparation and the pyrolysis condition in the production of biochar.

In this paper, providing an overview of the biochar adsorption on soil and the mechanism about the biochar adsorption of the fertilizer, the heavy metal and the organic pollution, we aim to explore the better use for biochar in practices. We will also give some research directions associated with the problem of using biochar.

Applied Mechanics and Materials Vols. 448-453 (2014) pp 417-424Online available since 2013/Oct/31 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMM.448-453.417

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Physical and Chemical Properties of Biochar

Biochar Properties

Biochar is the product of thermal degradation of organic materials, such as Wood, straw, herbs,

and manure，in the absence of air or with low oxygen (pyrolysis)[4]. Feedstock currently includes wood chip and wood pellets, treebark, crop residues (straw, nut shells and rice hulls), switch grass, organic wastes including distillers’ grain, gas from the sugarcane industry, chicken litter, dairy manure, sewage sludge and paper sludge[5]. While as, the material properties and structures of biochar are little different due to the raw material and the environment in pyrolysis. Generally speaking, biochar composition can be crudely divided into relatively recalcitrant C, labile or leachable C, a small amount of minerals and volatile organic compounds[6]. Meanwhile, there is a large surface, which have net positive and net negative surface change, causing by a lot of pore structure inside[7]. The power of adsorption is drive by the properties of biochar. This section gives a brief overview of the unique properties of biochar compared to other compounds in soil as a background to following section that discuss the adsorption effects of biochar in soil.

The Factors Affecting the Properties of Biochar

The effects of biochar adsorption on the soil might be driven by the difference in the physical properties and its chemistry between biochar and soil，while the raw material used for pyrolysis and the condition such as the temperature[8] and oxygen content in the pyrolyzer play important roles in determining the properties of the biochar[9]. A range of process conditions, such as the composition of the feedstock, temperature and the heating rate , the properties of biochar have provide different amounts and properties of products[10]. (tab.1).

Tab.1 Yields and some properties of biochars from different substrates and reaction conditions

The Raw Material for Biochar. Biochar is the carbon rich product obtained with the gaseous state(CO2, combustible gas, volatile oil and tar) removed, so the related researches have demonstrated that the content of nutrient elements in biochar and its preparation of feedstock into a linear correlation.

Compared four leguminous plants (beans, broad beans, mung beans, peanuts) with rape, wheat, corn, rice straw and rice husk, Yuan[11] and Xu[12] found that the biochar from the leguminous plants with the higher content of Ca, Mg, K still have much more Ca, Mg, K. the biochar produced from wood biomass always have high total C content, low ash, low mineral composition(N, P, K, Ca and so on) and low cation exchange capacity(CEC). In some experiment, they added the biochar produced from broad-leaved tree and needle-leaved tree soil respectively and found that the former

Biochar source Willow Pine Miscanthus Miscanthus Miscanthus

Pyrolysis time, min 10 10 10 10 60

Temperature 500 500 500 400 600

Yield andchar, wg% 25.0-26.2 22.2-22.5 25.9-26.2 29.4-31.2 19.8-20.2

Surface area, m2/g 1.41–1.55 1.93–2.15 1.65–1.95 1.4–1.7 50.9–51.1

C wg% 79.9 81.4 76.3 74.8 85.1

H wg% 3.34 3.38 4.26 4.33 2.40

N wg% 0.97 0.47 0.40 0.39 0.55

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biochar is much more effective in raising the PH value and increasing CEC of soil[13]. While, depending on the mineral composition, the ash, total C content and so on which decide the ability of biochar adsorption, the feedstock plays an important roles in biochar adsorption.

The Pyrolysis Conditions. Depending on the operational time and temperature, there are three subclasses of pyrolysis. Yields of liquid products called biochar are maximized in conditions of low temperature, high heating rate, and a short gas residence time, whereas a high temperature, low heating rate and long gas residence time would maximize yields of fuel gas. Low operational temperatures and low heating rates give maximum yields of biochar[14].obviously, the yields of the biochar or the by-product( biofuel) depend on the temperature in the pyrolysis on a large number, at the same time, which determines the properties of the biochar[15]. A research shows that, the higher in temperature, the lower yields of the biochar provided by the crop stalks, but the higher C concentration in the produce[16].

Carbonate crystals and organic anions with the oxygen functional group such as-COO-(-COOH) and-O-(-OH) make the aqueous solution of biochar show alkalinity[17]. In a research, to obtain the biochar using the rape straw, corn straw, soybean straw and peanut straw with anaerobic pyrolysis method in 300, 500 and 700 ℃ respectively, Yuan and XU[18] found that the alkali content and pH value of the biochars were increased with the temperature increasing in p. In other study, they[19] found that as another existence of alkalinity, the content of the organic anions decreased with the rise of the temperature in process. Some research[20] found that porosity of biochars, provided by dehydrated sludge and corn stalk, enhanced with the rising of temperature on the 300 ~ 900 ℃ temperature range. Meanwhile, the biochar from the corn stalk have a lower porosity with the temperature rising beyond 900℃,and the micro pore porosity and small pore number reaches the maximum on 900℃[21].

Biochar Sorption on Soil

The Effects of Biochar Sorption on Water Holding Capacity and Fertilizer in Soil

Soil moisture content is primarily determined by soil texture and precipitation rate. As yet there is little clear experimental proof that biochar does impact on soil or plant water relations. It has also been suggested that if biochar contains sufficient amounts of humic substances, it can increase soil water holding capacity. As know, with the application of biochar, there will be changes in soil pore-size distribution and this could alter percolation patterns, residence time and flow paths of the soil solution. An experiment[22] has proved that due to the much more pore structure, larger specific surface area, and lower bulk density of biochar compared with soil, the biochar applied in soil can improve the water holding capacity of soil. In a research, the added biochar has improved the pore structure of biochar and decreased bulk density of soil so that the water-holding capacity was increased. At the same time, as the result of the reduce in water-percolation rate by biochar pore structure, the residence time of nutrient in soil has been prolonged and the sorption power was been enhanced.

If water holding capacity is increased then expectations of nutritional benefits could also be gained for soil solution mobile elements. In a study[23], the researcher has found that the added biochar can greatly reduce the leaching of nitrogen in the soil. Compared to nitrate nitrogen, the biochar was much more available to organic nitrogen. Based on research by Lehmann et.al[24], with the fertilizer added in biochar, the biochar sorption of NH4

+ in soil has been enhanced significantly and the nitrogen utilization rate by rice has raised a lot. As has already been mentioned

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the capacity of biochar and its potential to both adsorb N (NH4+) and subsequently increase

N-availability to the plant may explain varied yield responses; this is a key question that warrants further experimentation.

While N sorption rised, Nutrient bioavailability and plant uptake of K, Ca have in some cases increased in response to biochar application. A lot of experiments have proved that the biochar characters determined the unique biochar sorption in nutrient. The biochars composed of much more K+, Ca2+, Mg2+ have much lower capacity of adsorption in K+, Ca2+, Mg2+[25]. The biochar produced from pecan shell can raised the soil sorption of Ca, K, Mn and declined these matters leaching[26].

Opportunities to conserve soil moisture content and enhance the nutrients available for plant extraction will be a critical factor in sustaining agricultural production. It is also likely, should biochar provide a route by which soil moisture content can be conserved , the nutrient can be stored and controlled-released so that it could be a useful tool in landscape landscaping organic mulch particularly in more sandy soils (arid regions) where soil moisture retention is critical in the initial reestablishment of vegetation and crops or a practical carrier for fertilizer in poor soil where biochar can help the utilization rate of fertilizer.

The Effect of Biochar on Heavy Metal in Soil

Adsorption of heavy metal in biochar from the soil could involve a number of possible mechanism, including ion exchange, chemical exchange, surface precipitation and electrostatic attraction etc. In a study[27] of relative distribution of Pb2+sorption mechanisms by sludge-derived biochar, the authors introduced some possible mechanisms for Pb2+ sorption by the biochar:I) heavy metal exchange with Ca2+, Mg2+, and other cations associated with biochar, attributing to co-precipitation and inner sphere complexation with complexed humic mater and mineral oxides of biochar; II) the surface complexation of heavy metals with different functional groups, and inner sphere complexation with the free hydroxyl of mineral oxides and other surface precipitation; III) the physical adsorption and surface precipitation that contribute to the stabilization of Pb2+. Moreover, some metal ion are attracted by the surface charge of biochar ,but in this reaction only some metal ions is available, for instance of Pb2+, Cu2+. But to some metal ions, the carbonate and oxygen-containing functional group in the surface of biochar play important roles in sorption of heavy metal. Therefore, it is conceivable that under realistic field situation, sorption mechanisms for heavy metal by biochar could be dependent on the type of soils as well as types of heavy metals.

Many studies have proved that the application of biochar into contaminated soil could reduce the sorption of heavy metal by plant as to improve the soil environment[28]. An experiment[29] carried out in a contaminated soil demonstrated that the Cr content of soil pore water was reduced 10 times by the application of biochar. And others[30]have proved that with the biochar produced from cotton stalk, the Cr accumulation in the edible part and the roots of Chinese cabbage have reduced 49.43%-68.29% and 64.14%-77.66% respectively. Wang Weihang[31] et al. have used the modified nano-biochar into improve the contaminated soil by heavy metal , and found that with biochar application rates in soil, the reduction of the available Cu, Zn content of soil was increased sharply. Meanwhile, some findings showed that the surface functional group of biochar can enhance the capacity of soil in heavy metal such as Ni2+, Cu2+, Pb2+ and Cd2+ .

However, Namgay et.al[32] added an activated wood biochar into a soil to investigate the effect of biochar on the As, Cd, Cu, Pb, and Zn to maize, and found that the concentration of As, Cd, and Cu in maize were decreased and the Pb and Zn were inconsistent in the shoots. Therefore,

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Biochar can reduce the bioavailability of the metal pollution through sorption, and reduce the risk of the pollutants entering human food chain or leaching to groundwater. However, in other words, the metals are gathered in soil .The long-term environmental fate of the sequestered contaminants is still unknown, and further research is needed.

The Effect on Organic Material of Ssoil

The organic pollutants in soil are usually caused by a wide range of industrial activities, farming practices, and inadequate application of wastes. These compounds maybe participate in the course of Adsorption/desorption, volatilization, percolation, biodegradation, the biodegradation, but the adsorption plays great important roles in the pollution caused by organic compound[33].

Biochar has been reported to be very effective in adsorption of many natural and anthropogenic organic compounds. Many past studies have demonstrated that given the highly aromatic nature, high surface area, micro-pore volume, and the presence of abundance of polar functional groups in biochar, the material has been found to be effective in the uptake of a variety of organic chemicals including pesticides, PAHs, and emerging contaminants such as steroid hormones. The sorption of biochar in organic pollution contributes to keep the pollutions in soil from the plants and ground water as to avoid the pollution translocation. However, in other words, the pollutions absorbed by the biochar are accumulated as to accelerate soil contamination.

A experiment[34] carried in greenhouse has proved that the application of biochar can significantly reduce the accumulation of polycyclic aromatic hydrocarbon (PAH) in plants growing in the sludge soil system. Meanwhile, a study[35] on the influence of biochar inputting on the adsorption of phenanthrene by soils and by maize seedlings demonstrated that the added biochar remarkably enhanced the adsorption of phenanthrene and reduced the adsorption by the seedlings. However, Biochar adsorption of organic pollutants in the soil is greatly influenced by the feedstocks and pyrolysis condition (such as temperature), which change the surface area and the amount and type of functional groups in the surface of biochar. A study[36] has proved that the biochar pyrolyzed at 700 °C had a much higher adsorption capacity but weaker desorption capacity of terbuthylazine in soils than the biochar produced at 350 °C. Wang Tingting[37] et.al has found that added into different content organic materials, the adsorption of pesticide were enhanced respectively but not to be in one lever, and the biochar adsorption was reduced by the organic materials.

Conclusions and future research

Biochars produced from different biomass materials and with different pyrolysis conditions present highly heterogeneous physicochemical properties with different adsorption ability, which can affect the efficacy in impact on kinds of soils. Biochar has the potential to be developed as ab effective technology for keep the water and fertilizer from soils. We can use biochar as landscaping organic mulch and also it can be developed as substrate to control the release of fertilizer added to soil. Meanwhile, the ability in adsorption of heavy metal and organic pollutants in soil, it can be used as a potential technology for remediation of contaminated agricultural soils. But so many aspects are still yet to be investigated further.

First of all , the sorption mechanism of biochar has not yet to be mastered well, but based on the laboratory and small plot trials, large-scale field trials are essential before operational scale projects are implemented.

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Secondly, the ability and capacity in adsorption of biochar vary with different feedstocks and conditions in pyrolysis. So it is important to optimize production systems to produce designer biochar products based on the using effectively for special work.

The last one, with more stronger absorb of heavy metal and organic pollution than soil, biochar added into soil can avoid the pollutions into the plants. In other words, the biochar can slow down these pollutions into cycle of ecosystem rather than remove pollutions. Whether or not it means the pollutions are enriched cased by the biochar application in the amended soil. Therefore, since a lot of trials demonstrated that biochar was friendly to environment, we still don’t know what it would bring to soil after hundreds of years.

Acknowledgements

This paper was sponsored by National Engineering Research Center of Protected Agriculture(2011BAD43B00) and key scientific research project of Shanghai (Project No. 11DZ1960204) and Fundamental Research Funds for the Central Universities.

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