ReviewThe adsorption, regeneration and engineering applications of biochar for removal organic pollutants: A review
Introduction
In recent years, biochar has gradually entered people's vision. Biochar is a carbon-rich solid, which is obtained by heating biomass in an oxygen depleted environment, such as wood, manure with little or no oxygen. And biochar can endure in soil for thousands of years (Liu et al., 2018a, Liu et al., 2018b; Tan et al., 2015). The word “biochar” is a combination of “bio-” as in “biomass” and “char” as in “charcoal”. It has been used in scientific literature ever since (https://en.wikipedia.org/wiki/Biochar).
Biochar is characterized by large specific surface area, porous structure, abundant surface functional groups and mineral components, which makes it be used as proper adsorbent to remove pollutants from aqueous solutions possibly (Zhang et al., 2018; Zhu et al., 2018). As a kind of adsorbent, biochar, with porous structure similar to activated carbon, is the most commonly used and effective adsorbent in the world to remove various pollutants in water (Wu et al., 2018). However, different from activated carbon, biochar is a new kind of adsorbent with low cost and high efficiency (Tan et al., 2015). The production of activated carbon requires higher temperature and activation process. Biochar, by contrast, is cheaper to produce and uses less energy. Biochar is a renewable resource, and is an ideal resource for environmental technologies for water pollutant treatment due to its economic and environmental benefits (https://www.biochar-international.org/sustainability-climate-change/).
With the development of biochar research, people gradually find out the great environmental and economic benefits it brings to us and conduct scientific research on biochar to apply it to engineering (Bian et al., 2018). The research on biochar has also been initiated gradually in various countries. The research of the biochar has been getting more and more attention from 2005 to 2015. The scale and use of biochar systems around the world may vary greatly, which depends on raw materials, intended uses of biochar, production technologies, local economies and the environment (Tan et al., 2015). The increasing research in biochar has inaugurated multidisciplinary areas science researches and engineering applications. A large number of studies have displayed the benefit of development of biochar on mitigating global warming (Koga et al., 2017), soil amendment (El-Naggar et al., 2018a, El-Naggar et al., 2018b; Liu et al., 2018a, Liu et al., 2018b), enhancing of crop yield (Al-wabel et al., 2017) and carbon storage (El-Naggar et al., 2018a, El-Naggar et al., 2018b; El-Naggar et al., 2018a, El-Naggar et al., 2018b). At present, the scientific research on biochar application is mainly on soil (Li et al., 2018b, Li et al., 2018a, Li et al., 2017b), water (Saarnio et al., 2018), climate-related aspects (Nair et al., 2017), pesticides (Ćwieląg-Piasecka et al., 2018), antibiotics (Zeng et al., 2018), heavy metal (Liu et al., 2018a, Liu et al., 2018b) and fuel (Waqas et al., 2018). For instance, some of the most commonly benefits of the application on soils include: increased long-team C storage in soils, restored soil fertility, improved soil physical properties, improved crop yield and nutrition, enhanced retention of environmental pollution, and reduced N-emission and leaching (Morales et al., 2015). As a result, biochar has become more and more popular in many types of applications as an adsorbent, soil modifier and climate change mitigation method.
As an adsorbent, biochar can absorb organic pollutants well. At present, many researchers have studied the adsorption of biochar on soil organics such as polycyclic aromatic hydrocarbons (Song et al., 2017) and phthalates acid esters (He et al., 2016). Some studies have also shown that the treatment effect of sewage treatment system containing biochar on organic pollutants is significantly improved compared with that without biochar (Sun et al., 2018). Therefore, biochar has great potential. Biochar has a variety of applications. So far, some researchers have comprehensively described the relationship between the properties of biochar and raw material selection and technological conditions by integrating the results of a large number of biochar production experiments and related literature (Weber and Quicker, 2018). Through the comprehensive study of biomass dry and wet heat transfer in recent years, integrated research on biomass dry and wet heat transfer in recent years, integrated various properties of biochar generated under different conditions, and integrated application of biochar as adsorbent (Gan et al., 2018). Some people have also summarized the properties of biochar to improve the removal rate of target pollutants in rainwater as well as the challenges and opportunities for rainwater treatment by studying the mechanism of biochar removal of pollutants (Mohanty et al., 2018).
Previous studies on the nature and application of biochar have been conducted, and some people have reviewed the nature and application of biochar. However, little research has been done on the engineering application about the treatment of organic pollutants and regeneration of biochar in recent years. In this paper, a large amount of research literature on adsorption of organic pollutants by biochar in recent years is reviewed to provide a summary of the latest information on biochar adsorption of organic pollutants and its application in engineering and regeneration. This article introduces the related knowledge of biochar (the basic physical and chemical properties, sources, types, production mode), integrate the interaction mechanism of oil pollutants, pesticides, antibiotics, fuel, persistent organic pollutants (POPs) and biochar, enumerates the biochar application in engineering and regeneration methods of sorting the biochar. For example, if the wastewater containing phenol was treated in a fixed bed, biochar was filled in to absorb the phenol in the wastewater as an adsorbent (Shao, 2013; Shao et al., 2013). Biochar was partially added to the fluidized bed to remove organic vapor (Aragón et al., 2009). Biochar was added to the soil to absorb pesticides in the soil (Trigo et al., 2016).
Section snippets
Source and type
Biochar should be made from biomass waste. Biomass waste materials appropriate for biochar production including crop residues (both field residues and processing residues such as shells (Xu et al., 2018; Zazycki et al., 2018), fruit pits (Aggelopoulosa et al., 2017), bagasse (Nie et al., 2018), as well as food and forestry wastes (Cao et al., 2018; Gupta et al., 2018), animal manures (Al-wabel et al., 2017) and sludge (Vause et al., 2018). The raw materials of biochar mainly come from
Remove organic contaminants
Organic pollutants are an important part of environmental pollutants and have seriously threatened the ecological environment and human health (Jiang et al., 2018). Organic pollutants are more extensive and complex than heavy metal pollution (Cui et al., 2018). Environmental organic pollutants come mainly from two aspects, natural sources and anthropogenic sources. Environmental organic pollution is the result of a combination of various sources. Important sources of pollution include pesticide
Regeneration
Biochar regeneration is an inverse process of adsorption. There are two principles of regeneration: adsorbate desorption and adsorbate decomposition (Jia et al., 2016). A competitive adsorbent should express a good reusable and recycling ability for industrial applications and could reduce the cost of biochar sorbents greatly as repeated sorption-desorption cycle (Ifthikar et al., 2018; Wang et al., 2015). After grinding the superfine magnetic biochar adsorbent, the tetracycline itself degraded
Application in liquid phase
The most widely used treatment for fixed bed is for the treatment of phenol containing wastewater (Shao, 2013). The structure of fixed bed is shown in Fig. 5. Coming from fixed bed, structured fixed bed uses a gradient loading method to fill two or more adsorbents in the bed. Common structured fixed bed is to fill a certain proportion of adsorbents with large adsorption capacity at the inlet, and a certain proportion of adsorbents at the outlet can enhance mass transfer efficiency and reduce
Analysis of economic benefit
In the case of biochar-based products, biochar costs depend on local raw material availability, pyrolysis conditions, process flow, reactor availability, and recovery and service life issues. At the same time, biochar may be a by-product of some processes (Mohan et al., 2015). The International Biochar Initiative (IBI) and the European Biochar Certificate have been working on the development of biochar standards in recent years. In 2013, IBI survey of biochar sellers found that global biochar
Expectation
As an adsorbent with wide sources and good adsorption performance, biochar has broad prospect in the removal of organic pollutants in the future. However, there is a lack of systematic research on biochar including the mechanism of its adsorption of organic matter and the applied theory, the treatment for possible secondary pollution in regeneration, and the application of specific functional biochar (Wang et al., 2013; Zhang et al., 2017a, Zhang et al., 2017b).
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Biochar for specific purposes can
Conclusions
With the continuous development of industry, the extensive development model of “high energy consumption, high pollution and high emissions” has led to increasing environmental pollution. Organic pollution has caused serious problems to water and soil. Biochar is gaining attention as a low-cost, high-efficiency, renewable adsorbent. The surface chemistry of biochar is affected by the calcined material, pH, temperature and other factors, and the removal mechanism for different organic
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