Investigation into lanthanum-coated biochar obtained from urban dewatered sewage sludge for enhanced phosphate adsorption
Graphical abstract
Introduction
The significant use and excessive discharge of phosphorus increase nutrient concentration in local streams, accelerates the growth of blue-green algae, and causes fish death. Such problems will continue to deteriorate and endanger human life if there were no proper solutions. What is more, the tighter phosphorus discharging limits (e.g. he permissible phosphorus discharge concentrations in wastewater treatment will decrease from 1 to 2 mg/L to 0.1 mg/L in the European Union under the Water Framework Directive, Shepherd et al., 2016) requires a substantial modification of current wastewater process. However, current phosphorus removal technologies such as chemical precipitation and biological treatment are unable to satisfy new standards (Wu et al., 2017).
Adsorption is a preferable approach for its simplicity of design and good performance even at low concentrations (Loganathan et al., 2014). It is vital to develop an environmental friendly adsorbent with no secondary pollution (Li et al., 2016; Kong et al., 2018a, Kong et al., 2018b; Yuan et al., 2020). For example, natural materials (such as dolomite, sepiolite) (Yin et al., 2011; Boeykens et al., 2017); synthetic metal oxides and hydroxides (such as ZnAl LDH, FeMn binary oxide) (Lu et al., 2014; Hatami et al., 2018); functionalized inorganic materials (such as chitosan/La hydroxide composite aerogel beads) (Lin et al., 2018) have been examined for phosphate adsorption. In recent years, biochar with various metal oxides or hydroxides, especially magnesium, iron, aluminum and calcium, have received considerable attention because of their superior phosphate adsorption capacity. For example, the maximum phosphate adsorption capacity of biochar prepared from Mg-enriched tomato tissues could reach 100 mg/g (Yao et al., 2013). Biochar with 20% Al content exhibited an optimal phosphate adsorption capacity at 57.49 mg/g (Yin et al., 2018). The phosphate adsorption capacity of FeCl3-impregnated biochar was reported to be 111.0 mg/g (Yang et al., 2018).
Besides, La-containing materials have attracted more and more attention for phosphorus adsorption due to its superior phosphate affinity, excellent selectivity, and wide operating pH range (Huang et al., 2014). Many materials, such as silica spheres (Huang et al., 2015), porous zeolite (He et al., 2017), tourmaline (Li et al., 2015) and magnetic mesoporous nanospheres (Chen et al., 2019), have been investigated as a supporter for La oxides or hydroxides. In previous literature, one gram La2O3 modified oak takes up 46.37 mg phosphorus (Wang et al., 2016), and La(OH)3-modified magnetic pineapple biochar absorbs 101.16 mg/g (Liao et al., 2018). To evenly distribute La over the surface, the preferred supporting materials should have large surface areas and abundant pores.
In China, wastewater treatment plants generated over 60 million tons of sewage sludge (80% water content) each year, in which the various heavy metals, organic micropollutants and pathogens might threaten human health (Fytili and Zabaniotou, 2008). It is valuable to convert the sewage sludge, a hard-to-handle solid waste, into effective adsorbents due to its low cost and easy access of the material (Li et al., 2019). However, various heavy metals in sewage sludge might be bound to organic material and coprecipitate (Wang et al., 2019). Previous literature reported that most heavy metals are present in the carbon matrix in the oxidizable and residual forms after pyrolysis, especially at 600 °C, reducing its potential ecological risks (Jin et al., 2016). However, it is still necessary to consider the risks of sludge-based biochar during phosphorus adsorption.
Therefore, this research is to develop a sewage sludge-based phosphate adsorbent. A variety of metals, including magnesium, calcium, aluminum, iron and lanthanum, have been compared to maximize the phosphate adsorption capacity under different surface treatment processes. The optimum loading condition and the method for further improving the adsorption capacity were also investigated. Meanwhile, the leaching experiments of different sludge-based biochar were performed to test whether the synthetic biochar will become a secondary pollutant or not.
Section snippets
Materials
The dewatered sewage sludge was collected from the North Municipal Sewage Treatment plant, in Dongguan, China. It was air-dried at 105 °C until the moisture was completed removed. The dried sludge was then ground and passed through an 80–100 mesh. The obtained fine powder was then heated in a programmable tube electric furnace (BTF1200C, Anhui BEQ Equipment Technology Co. Ltd., China) with a temperature increase of 5 °C /min, and held at the target temperature for 2 h surrounded by nitrogen
Effect of different metal-loaded biochar
Fig. 1 displays the phosphate adsorption amount using biochar modified by different metals under various treatment methods. As can be seen in Fig. 1A, direct sludge biochar addition into metal salt solutions have little effect on phosphate adsorption, suggesting that direct immersion could not effectively load metal ions onto the biochar. Furthermore, the pretreatment process through acid and alkali conditioning were carried to increase the porosity of the biochar and to explore the effect of
Conclusion
A highly efficient phosphate adsorbent, where La compounds were loaded onto the sludge biochar, was prepared and tested. The results indicated that the La-600SS-OH had the highest phosphate adsorption capacity (93.91 mg/g). The adsorption process was well described by the pseudo-second-order and the Langmuir isothermal adsorption model. The adsorbent had better performance at a pH range of 3.0 to 6.0, while other common anions in wastewater had a negligible effect on the phosphate adsorption.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the Natural Science Foundation of Hebei Province (Grant nos. E2018203293) and Qinhuangdao Science and Technology Research and Development Plan (Grant nos. 201801B031).
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