Influence of sodium hydroxide addition on characteristics and environmental risk of heavy metals in biochars derived from swine manure
Introduction
The annual production of livestock and poultry manures in China is estimated to be about 3.2 billion tons (Zhang et al., 2012). The conventional practice of direct application of animal manures into farmland has been found effective to improve soil fertility and increase crop yield (Lupwayi et al., 2005, Mandal et al., 2007). However, the amount of heavy metals in animal manures is generally high, originating from feedstock additives for disease prevention and livestock and poultry growth promotion (Yang et al., 2017). More seriously, long-term applications of animal manures bearing high levels of heavy metals can lead to elevated concentrations in soils, which subsequently harm crops and humans when passed through the food chain (Xu et al., 2013, Xu et al., 2015, Hamidpour et al., 2016). Therefore, alternative and safe ways to use animal manures are needed.
Pyrolysis has been shown to reduce the volume of manure, decompose organic pollutants and kill pathogens (Troy et al., 2013), while producing bio-energy fuels (e.g., gases and bio-oils) and high-quality biochar (Monreal and Schnitzer, 2011, Clark et al., 2017). Additionally, recent studies have indicated that pyrolysis could immobilize heavy metals into the biochar matrix and transform them into a residual form (which is less likely to leach out), thereby reducing the potential risks of heavy metals (Meng et al., 2017, Lin et al., 2013, Lin et al., 2017, Zeng et al., 2018). However, these studies also revealed that low pyrolysis temperature did not effectively drive the distribution of metals speciation in biochar, and concluded that pyrolysis could enhance the stability of metals only when the temperature was high enough, meaning that it will consume a lot of energy. Hence, new pyrolysis technologies are needed to compensate for this shortcoming.
An addition of alkaline catalysts during the pyrolysis of animal manure may be a better choice to enhance immobilization efficiencies of heavy metals. Previous studies have shown that CaO/Fe2O3 could react with heavy metals in the sludge at high pyrolysis temperature (>500 °C), leading to the formation of new mineral phases (Chen et al., 2015, Sun et al., 2018, Liu et al., 2019). Furthermore, the addition of KOH promoted the residual fraction of Cd, while the stable fractions of Cu, Zn, Cr, Ni and Mn were converted to relatively unstable fractions during pyrolysis treatment of sewage sludge at 700 °C. This is likely due to further decomposition of inorganic minerals (mainly SiO2) induced by high amounts of KOH (20–50%, w/w), resulting in more metals being released (Li et al., 2018). In contrast, Yuan et al. (2011a) found that the NaOH additive (5%, W/W) led to the increase of stable fraction of Cu, Cu and Cr, whereas the opposite observation occurred to Pb and Cd during thermochemical liquefaction of sewage sludge at 320 °C. The comparison between above two results suggested that low amounts of alkali might be more suitable for immobilization of heavy metals during pyrolysis of sewage sludge. However, no information is available about effects of adding alkali on the transformation and environmental risk of heavy metals in biochar derived from animal manures.
In this study, we focused on swine manure (SM), because it contains high heavy metals compared with other manures (Yang et al., 2017). We selected sodium hydroxide (NaOH) as the alkali because it is less corrosive and more economic, in comparison to potassium hydroxide (KOH) (Cazetta et al., 2011). The main objectives were to (1) investigate the effect of NaOH addition on characteristics of biochar derived from SM, (2) explore the influence of NaOH addition on the transformation of heavy metals speciation during pyrolysis of SM, and (3) assess the potential ecological risk of heavy metals from biochar in the environment.
Section snippets
Sample collection and biochar preparation
Swine manure was obtained from a swine farm in the Shijiazhuang, Hebei Province, China. Samples were dried in an oven at 105 °C to a constant weight, and then pulverized to a size ≤ 0.15 mm. SM samples (100 g each) were mixed with dilute NaOH solution (200 mL) to reach the desired impregnated ratios (the mass ratio of NaOH to SM, 0.5% and 2%). The slurry was maintained for 24 h at room temperature, and then dried in an oven at 105 °C to a constant weight. Above feedstocks were placed into a
Physicochemical properties of SM and biochars
Properties of SM and biochars, including yield rate, ash content, pH, EC and elemental composition are exhibited in Table 1. Increasing pyrolysis temperature led to a decrease in yield rate of biochar, ranging from 64.47% to 48.32%, but a distinct increase in ash content of biochar. This was mainly due to increased decomposition of organic matter under elevated temperature, which resulted in the enrichment of mineral in solid residues (Cantrell et al., 2012). When NaOH was added to SM, a slight
Conclusions
This study demonstrated effects of NaOH on immobilization of heavy metals in the biochars derived from swine manure and their properties. The pH, EC, ash content, yield rate, aromaticity, and hydrophily of biochar were increased by NaOH addition, whereas surface area was reduced. The addition of NaOH pushed the unstable fraction (F1 + F2) of Cu, Zn, Cr, Pb, and Cd to relatively stable (F3) or stable (F4) states through the formation of more stable compound. As such, leachability and potential
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.
Acknowledgements
This work was supported by Huai'an Research Program of Basic Research and Frontier Technology (NO. HABZ201802).
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