Regular ArticleEffect of pyrolysis temperature on phosphate adsorption characteristics and mechanisms of crawfish char
Graphical abstract
Introduction
Crawfish (CF), also known as crayfish, is a species of crustaceans living in freshwater and is taxonomically classified as Astacoidea and Parastacoidea [1]. In the U.S., CF is a typically produced in Louisiana, and its total annual yield ranged from 10,000 to 27,000 tons during last decade [2], [3]. The dishes made with CF are highly popular with people all over the United States particularly in the Southern region, and CF dishes are usually enjoyed from April to June. In general, many people eat only the tail meat, which is only 15–20% of the total crawfish weight [4]. Considering the annual yield and the percentage of consumed tail meat, the amount of CF waste generated annually is expected to be substantial. However, the remaining CF waste is discarded with general waste without any post treatment.
Despite the efforts of many researchers to recycle CF waste for decades, known useful techniques are relatively poor. Examples of successful studies include the production of chitin and chitosan from crawfish waste through acid and alkali treatments [5], [6], and the extraction of carotenoids (astaxanthin) through solvent extraction [7]. However, these techniques are costly and time-consuming to utilize, and the amount of CF waste that is recycled by these technologies is extremely limited. Therefore, the development of new economical and creative technologies to effectively recycle CF is urgently required.
The pyrolysis technology of the waste has attracted attention from many researchers because the treatment process is simpler than other technologies and it can be applied to a large amount of waste [8]. In addition, pyrolysis technology to treat waste resources is effective in reducing waste, and chars derived from waste resources are known as an effective adsorbent for various pollutants [9]. Recently, it has been reported that char produced from pyrolysis of agricultural residue, sewage sludge, food waste, animal manure and bone is effective for the adsorption of various contaminants such as heavy metals, PHAs, pesticides, dyes and PO43− [9], [10], [11]. However, since the adsorption characteristics of char for treating pollutants depend on the characteristics of raw materials and pyrolysis conditions, the optimum pyrolysis temperature and raw material selection are essential for utilizing char as an adsorbent [12]. It is also important to select an adsorbent suitable for pollutant because adsorption characteristics of adsorbents vary depending on the pollutants in the wastewater [13]. In particular, char produced from pyrolysis of crustacean waste contains a large amount of calcium, suggesting that it could be utilized as an adsorbent of phosphate in wastewater.
Phosphate is known to be discharged to nearby water systems through a variety of routes such as leaching and runoff of fertilizers used in agriculture, household detergents, and chemical treatment process of industrial facilities [14]. Phosphates are essential for the growth of aquatic organisms and plants, but excessive phosphate concentrations in waterbodies induce the growth and proliferation of phytoplankton, leading to eutrophication. It also has a negative impact on aquatic organisms and animals [15]. Since eutrophication occurs mainly when phosphate concentrations are above 0.02 mg/L in waterbodies [16], wastewater treatment plants use chemicals such as Al and Mg salts and adsorbents to treat phosphate below required standards. However, the unfavorable economics of using treatment chemicals and the post-treatment sludge produced are problematic. On the other hand, in the case of using adsorbents, low phosphate adsorption efficiency requires that the adsorbent be replaced frequently. Therefore, utilization of calcium rich-char obtained from pyrolysis of CF as a phosphate adsorbent could be a breakthrough technology that can improve waste reduction as well as phosphate treatment efficiency.
Recently, it has been reported that char produced from a crab shell is effective for phosphate adsorption [17], and some researchers emphasized the mechanism of phosphate adsorption by adsorbent containing a large amount of calcium [18], [19]. However, there has been no study on phosphate adsorption by crawfish char. Furthermore, although adsorption of heavy metals and organic contaminants by crayfish char has been reported [20], [21], there is still limited information on the CFC properties as influenced by pyrolysis temperature and on the mechanism of pollutants adsorption. Therefore, the purpose of this study was to determine the optimum pyrolysis temperature for CFC production and to evaluate its phosphate adsorption characteristics.
Section snippets
Preparation and characterization of crawfish char
Crawfish waste was collected from region restaurant in Baton Rouge (LA, USA), and used to produce CFC with slow pyrolyzer. Briefly, CF waste was rinsed first for several times with deionized (DI) water to remove remained impurities such as meat and salt, oven-dried (60 °C), and ground to pass through a < 0.5 mm sieve. Dried CF waste was placed in porcelain crucibles with a cover and pyrolyzed at different temperatures (200, 400, 600 and 800 °C) for 2 h after it reached set temperature in a
Properties of crawfish char derived from different pyrolysis temperatures
Table 1 shows the characteristics of CFCs according to pyrolysis temperature. The yield of CFC was decreased from 83.1% to 35.3% with pyrolysis temperature increasing from 200 °C to 800 °C. According to An et al. [25], the chemical composition of crab shells is known to contain protein (29.19%), ash (40.60%), lipid (1.35%) and chitin (26.65%) on dry weight basis. In our study, the yield of CFC from 400 °C and above was drastically reduced, which is attributed to the degradation of protein, fat
Conclusions
The characteristic of CFC depends on the pyrolysis temperature, and the main element of CFC produced at high temperature was calcium. The adsorption of phosphate by CFC increased with increasing pyrolysis temperature, and adsorption maxima of CFC600 and CFC800 were 56.8 and 70.9 mg P/g, respectively. The dominant reaction for phosphate removal by CFC600 involved both the adsorption of phosphate through ion exchange mechanism of phosphate hydrolysis products (H2PO4−, HPO42−) with CFC surface and
Acknowledgments
This project was supported by the Louisiana Agricultural Experiment Station Hatch Project-LAB94152, Louisiana State University, Baton Rouge, LA, United States.
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