Research articleThe characterization of biochars derived from rice straw and swine manure, and their potential and risk in N and P removal from water
Graphical abstract
Introduction
Biochar, a solid material obtained from the thermochemical conversion of biomass in an oxygen-limited environment, has attracted widespread attention in environmental management and agricultural practices due to its potential benefits in carbon sequestration, treatment of contaminated water, soil amendments, and so on (Lian and Xing, 2017; Saadat et al., 2018; Ahmad et al., 2014). Extensive research has shown that biochar is an excellent carbonaceous sorbent for contaminants including organic or inorganic pollutants (Peiris et al., 2017; Zhou et al., 2017; Gwenzi et al., 2015). The main reasons why biochar was widely applied in environmental remediation are mainly because (1) biochar is cheap and can be produced from many biomass wastes (Chen et al., 2018; Pratiwi et al., 2016; Gwenzi et al., 2017), (2) biochar usually has high surface area and large pore volume (Lian and Xing, 2017; Zhou et al., 2017), and (3) biochar can be briefly modified to enhance its removal efficiency of contaminants (Zhou et al., 2017; Yin et al., 2018a).
Until now, many organic wastes including agricultural residues, forestry wastes, animal manures and other materials have been used to produce biochars through pyrolysis at different temperature (e.g. 300–800 °C) (Chen et al., 2018; Mukherjee and Zimmerman, 2013; Yu et al., 2016; Gai et al., 2014; Yao et al., 2012; Liang et al., 2017). These obtained biochars showed different potentials in removing pollutants from water/soil environments (Lian and Xing, 2017; Ahmad et al., 2014; Peiris et al., 2017; Liang et al., 2017). Generally, the aromaticity, surface area and porosity of biochars are increased along with the increasing of pyrolysis temperature, although not always so (Lian and Xing, 2017; Yu et al., 2016). Meanwhile, the nature of the feedstocks is another factor regulating the characteristics of biochars. For example, several studies reported that the manure-based biochars demonstrated relatively higher ash content, pH and O/C ratio, as well as lower surface area than plant residues derived biochars (Wang et al., 2016; Zhao et al., 2016). According to literatures, it seems that biochars derived from animal manures were relatively less studied than plant residue biochars.
In fact, straw and manure are two of the most abundant biomass wastes in China (Chen et al., 2018). For instance, the annual production of rice straw is 0.12 billion tonnes, and the amount of swine manure is approaching 3.8 billion tonnes per year. Nowadays, the rice straw has been frequently used to produce biochar, and the derived biochar exhibited excellent ability to remove pollutants from water/soil (Liang et al., 2018a; Luo et al., 2011). However, swine manure has long been recommended for use as organic fertilizer although it may cause several environmental issues such as leaching of heavy metals and causing antibiotics pollution (Liang et al., 2018a, 2018b). To resolve these issues, it is necessary to find a suitable way to manage and use swine manure. Fortunately, it is found that the conversion of swine manure to biochar may not only provide a feasible way to resolve the abovementioned environmental issues, but may also have great potential to remove pollutants from water/soil (Chen et al., 2018; Liang et al., 2017, 2018a; Wang et al., 2016).
Nitrogen (N) and phosphorus (P), the major contributors to water eutrophication, has been excessively discharged into lakes and rivers due to the rapid development of industry and agriculture. In water systems, NH4+, NO3− and PO43− are the most common N and P form that mainly results in water eutrophication (Yin et al., 2018a; Cui et al., 2016). Therefore, great concerns have been given to how to efficiently and readily remove N and P from water. Among many treatment technologies, adsorption is proposed as the advantageous technology for N and P removal because of its economical and environmental friendly features (Yin et al., 2018b). Recently, biochar produced from different biomass has been frequently used to remove N and P from water, and demonstrated high removal efficiency of NH4+ and PO43− (Yin et al., 2018a, 2018b; Takaya et al., 2016). However, NO3− removal was not observed in several biochars (Gai et al., 2014; Hollister et al., 2013; Hale et al., 2013). It is worth noting that the feedstock of biochar can largely impact the sorption capacities or removal efficiency of N and P from water (Gai et al., 2014; Hollister et al., 2013; Hale et al., 2013).
The environmental risk of biochar has also become a growing concern due to the release of several contaminants (e.g., heavy metals, polyaromatic hydrocarbons, etc.) (Lian and Xing, 2017; Bastos et al., 2014; Luo and Gu, 2016; Huang et al., 2018; Qadeer et al., 2017). For instance, it is found that biochar can not only reduce the leaching of nutrients, but can also promote the loss of nutrients, especially PO43− (Pratiwi et al., 2016; Yao et al., 2012). As a consequence, the lost nutrients such as NH4+, NO3− and PO43− from soil might be another source of water eutrophication. Therefore, it is necessary to evaluate the removal efficiency of N and P by biochar as well as the risk of N and P release from biochar simultaneously before the large-scale application of biochar in environmental and agricultural practices.
So far, the information about the animal manure derived biochar to remove N and P is still scarce, and the difference of animal manure and plant residue derived biochar in removing N and P is relatively less explored. Hence, this study aimed to 1) compare the characteristics of animal manure (swine manure as representative) and plant residue (rice straw as representative) derived biochar, 2) investigate the removal efficiency of N and P by these biochars, and 3) evaluate the release of N and P from biochars for better understanding the safety and risk of biochar application in N and P removal from water.
Section snippets
Biochar production and modification
Rice straw and swine manure were collected from the experimental farm of Sichuan Agricultural University, Ya'an, China. These materials were air-dried and then ground through a 2 mm sieve before they were converted into biochar. The method of producing biochar was previously described (Chen et al., 2018). Briefly, certain amount of biomass was put into ceramic pot covered with lid to limit oxygen supply and then heated at 700 °C in a preheated muffle furnace for 2 h. The raw biochar derived
The characteristics of biochars
The physicochemical properties of biochars are shown in Table 1. The ash contents decreased in the order of SM (60.56%) > SMM (43.63%) > RS (41.01%) > RSM (26.15%), revealing MgCl2 modification reduced the ash content of both raw biochars. Moreover, the rice straw biochars had lower ash content than swine manure biochars under both raw and MgCl2 modified conditions. The pH values of four biochars ranged from 9.70 to 10.75. By comparison, the pH values of MgCl2 modified biochars were relatively
Discussion
Overall, the swine manure biochars (SM and SMM) generally had higher ash content, CEC value and N content, but lower pH, surface area, H content and O content than rice straw biochars (RS and RSM). On the other hand, MgCl2 modification reduced the ash content, C content, N content and surface area of raw biochars, whereas the pH, CEC, O content as well as pore size were enhanced. These differences or changes, especially the CEC, surface area and the pore size, might induce the different removal
Conclusions
In summary, the characteristics of swine manure biochar varied with rice straw biochar, and MgCl2 modification could change the surface area, porosity, pH and CEC of raw biochar. The application of swine manure and rice straw derived biochars to remove NH4+ was generally feasible, but the removal of NO3− and PO43− was unsatisfactory, especially PO43−. More importantly, NH4+, NO3− and PO43− were released from all biochars, suggesting biochars might become a possible source of N and P pollution
Conflict of interests
The authors declare that they have no conflict of interests.
Acknowledgments
This work was supported by Education Department of Sichuan Province, China [grant number 18ZA0373]; and the Agricultural Science and Technology Innovation Program (ASTIP), Chinese Academy of Agricultural Sciences.
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