Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
pubs.acs.org/journal/ascecg
Speciation of Nutrients in Hydrochar Produced from Hydrothermal Carbonization of Poultry Litter under Different Treatment Conditions Bashir M. Ghanim,† Witold Kwapinski,† and James J. Leahy*,† †
Carbolea Research Group, Department of Chemical Science, University of Limerick, Limerick V94 T9PX, Ireland
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S Supporting Information *
ABSTRACT: In this study, the effects of operating parameters on the behavior of nutrients during hydrothermal carbonization (HTC) of poultry litter (PL) were investigated. A number of HTC experiments were carried out using PL at different treatment temperatures, residence times, and initial pH. The standard measurement and testing protocol was adapted to determine the phosphorus (P) species in the solid hydrochar (HC), while the Ca, K, Mg, Na, Al, Zn, Mn, Fe, Cu, Cr, and Pb contents in each fraction were quantified and related to the P fractions. The results indicate that HTC can effectively reduce the solubility of most of the measured elements. The treatment temperature and initial pH can significantly influence the speciation of P and other nutrients, while the residence time effects were apparent at low treatment temperature. The majority of the measured nutrients remained in the HC when the hydrochar was produced without additional acid. Addition of acids, particularly H2SO4, provides a good approach for nutrient recovery. KEYWORDS: Phosphorus species, Treatment temperature, Residence time, Initial pH, CH3COOH, H2SO4, Water-soluble nutrients, Slow-release fertilizer
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INTRODUCTION
and composting of PL are common and are the cheapest methods for managing and disposal of it in large quantities. PL is also considered as an environmental pollutant, and several researchers have found that its application to agricultural land leads to soil P accumulation and acceleration of P losses to surface water.3,4 Consequently, because of the diminishing availability of land and stricter environmental regulations, the proportion of soil application and composting treatment facilities is gradually declining.6 Phosphorus (P) is essential for plant and animal growth with the vast majority derived from rock phosphorus, which is being slowly depleted3 and was added to the EU list of critical raw materials in 2014.7,8 Given the global concern with P resource depletion and to minimize the environmental risk of direct soil of PL and improve its utilization, the development of an
Livestock production is one of the fastest growing sectors of the agricultural economy driven primarily by growing demand for animal protein from an increasing global population. New production has shifted progressively from ruminants such as cattle which are fattened on grass and fodder to pigs and poultry fed on diets of feed concentrates. Poultry was responsible for 33% of the global meat production in 2010 and currently accounts for over 80% of the entire world’s livestock1 and is the fastest growing meat protein, the vast majority of which will be produced in industrialized, geographically concentrated intensive farming units.2 The accumulation of manure and animal related wastes often results in their over application to the fields as a nutrient source for crop growth, giving rise to social and environmental problems such as odors, pathogens, water eutrophication through leaching of phosphates to soil and surface water, acidification, and volatilization of greenhouse gases.3,4 Poultry litter (PL) is a type of biowaste rich in nutrients and organic matter.5 Thus, land application © XXXX American Chemical Society
Received: December 18, 2017 Revised: July 18, 2018 Published: July 23, 2018 A
DOI: 10.1021/acssuschemeng.7b04768 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
pH ranging from 7 to 2 achieved using H2SO4. The SMT protocol was applied to identify and quantify the different P species along with other nutrients in PL and its HC products. The concentrations of P and relevant elements in each fraction were determined using inductively coupled plasma optical emission spectrometry (ICP-OES). These HCs have been previously investigated for the effects of treatment temperature, residence time, and initial pH on yields and chemical properties of HC.14,15 The results obtained will help in understanding the effect of the examined treatment conditions on nutrient chemistry, specifically regarding P transformation in PL during HTC, and how to streamline the HC properties based on treatment conditions. Also, this work could provide insight into the fate of some inorganic elements during the HTC process.
alternative high-efficiency and environmentally friendly method for recycling of nutrients from the large quantities of PL is one strategy to achieve several goals simultaneously.3 Within the proposed Revised EC Fertilizer Regulations whose purpose is to equalize the market for products produced using recycled P and mineral fertilizers, the EU Commission undertook an assessment of potential candidate materials for inclusion in recycled fertilizer products, and the interim report nominated materials containing recovered phosphate salts, biochar, hydrochar, or incineration ashes as suitable candidate materials and provides technical guidelines for their quality. There is a growing interest in nutrient recovery from biowaste such as manures, and thermal processing has been proposed as a step to facilitate that, presenting an opportunity to mitigate several environmental issues in a single approach. A number of studies have demonstrated that thermal conversion processes such as pyrolysis, gasification, and hydrothermal carbonization may offer appropriate techniques for treating biomass/biowaste, producing a more stable product and enriching the product significantly with respect to P.9,10 Among the disposal alternatives, hydrothermal carbonization (HTC) is considered to be a very effective thermal treatment for high moisture biowaste and could become a promising strategy to control the nutrients loss problem.11,12 HTC is a thermal conversion process performed in water at moderate temperatures (180−260 °C) under self-generated pressure with residence times from a few minutes to several hours.13−16 HTC of PL gives rise to a solid carbon-rich hydrochar (HC) which can be used to recycle nutrients back into agricultural soil, along with aqueous and gaseous products.14,15 The chemical and physical properties of the HC produced are highly dependent on the process conditions such as treatment temperature, residence time, and initial pH.17,18 Manipulation of the treatment process condition is a major variable to produce suitable products as it can streamline the HC properties for a variety of uses.19−21 On this basis, it is important to understand the effects of the treatment conditions on the chemical and physical properties of the HC, particularly the immobilization and nutrient release from PL during processing.3 Even though considerable research has been conducted in the field of hydrothermal treatment,22,23 it is surprising given the abundant nutrient content in PL5 that few investigations have focused on the dependence of nutrient chemical speciation on the treatment conditions, with the vast majority of previous research reports targeting HC as a potential solid fuel. The behavior of nutrients, particularly phosphate, deserves more attention. Different methods can be employed for characterizing biowaste nutrient fractionation,24 including the standard measurement and testing (SMT) protocol which has been used for P fractionation in numerous biowaste samples;25−29 however, almost no information is available in the literature regarding the application of this method to understand the portioning of P in PL and its HC. The main objective of this study was to report on the changes occurring in the P species as well as other nutrients in the HC obtained from HTC of PL at different treatment temperatures, residence times, and initial pH. For this reason, a large number of HC samples was prepared at various treatment temperatures ranging from 150 to 300 °C for residence times of 5, 30, 120, and 480 min at the natural PL pH of 8.83 and at a treatment temperature 250 °C for 120 min at different initial pH, ranging from 9 to 4 achieved using CH3COOH or initial
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MATERIAL AND METHODS
Material. PL samples with straw as the bed material were collected from a farm located near Limerick, Ireland. The collected samples were transferred to the laboratory and prepared according to BS EN 14780:2011; they were kept in sealed polyethylene bags and stored in the freezer until required. The stored samples used for the experiments were on an as received basis (ar); however, prior to analysis, the PL was crushed and sieved to a particle size P > Mg > Na > Fe > Zn > Mn > Al > Cu > Cr ≈ Pb ≈ undetected. The same rank was obtained by Lynch et al.4 and Wang et al.30 who found that PL is rich in nutrients. However, other authors3,23,24 found that the Ca content is much higher than that of K. They attributed this difference to nutrient supplementation of the poultry feed. The results given in the Supporting Information Table S2 show that the K was found predominantly in the IP (26.6 mg g−1) and NAIP (18.3 mg g−1) fractions which were significantly higher than that measured in AP fraction (0.4 mg g−1); on the other hand, the highest concentration of the other metals (Ca, Mg, Fe, and Mn) was measured in the AP fraction. These observations indicate that the K phosphates have a higher solubility than those of the other elements measured which were consistent with the results of the watersoluble nutrients (WSN). As shown in Table 2, the watersoluble K (31.0 mg g−1) was much higher than either the water-soluble Ca, Mg, or Na, which were approximately 1, 3, and 4.4 mg g−1, respectively. The current values of P and Ca in PL were similar to those previously reported by Heilmann et al.;7 however, the concentrations of the other elements were
Figure 1. SMT protocol. the P species on the basis of varying solubility, which depends on the chemical form of the P.6,27 As seen in Figure 1, the protocol, which includes three separate extraction procedures, was originally designed to obtain five P fractions using HCl and NaOH as extractants. The first procedure was used for the total P fraction (TP) determined as an overall indicator; the second procedure was used for the inorganic P fraction (IP), which is mainly labile P (weakly bound to the sample matrix) and the organic P fraction (OP), while the third procedure was used for the apatite P fraction (AP), which is a stable form of P and assumed to be associated with Ca, and finally the nonapatite inorganic P fraction (NAIP), which is a moderately labile P and assumed to be associated with oxides and hydroxides of Al, Fe, and Mn, respectively.28 As shown in Figure 1, to obtain the TP fraction, ∼0.20 g (db) of each sample was calcined at 450 °C for 3 h, after which the cooled ash was stirred for 16 h with 20 mL of 3.5 M HCl; the mixture was subsequently centrifuged at 2000g for 15 min and filtered, and the TP was determined in the extract. To obtain the IP and OP fractions, ∼0.20 g (db) of each sample was stirred for 16 h with 20 mL of 1.0 M HCl and then centrifuged at 2000g for 15 min. The supernatant was filtered, and the extract was collected to determine the IP fraction while the residue was used to determine the OP fraction after washing twice with deionized water, stirring, and centrifuging at 2000g for 15 min. The dried residue was transferred to a crucible for calcining at 450 °C for 3 h; the cooled ash was stirred for 16 h with 20 mL of 1.0 M HCl and then centrifuged at 2000g for 15 min, and the supernatant was filtered. The extract was collected for OP analysis. To obtain AP and NAIP fractions, each sample ∼0.20 g (db) was mixed with 20 mL of 1.0 M NaOH, stirred for 16 h, and then centrifuged at 2000g for 15 min; the extract was used to determine the NAIP fraction, while the residue was used to determine the AP fraction. Ten milliliters of extract was mixed with 4 mL of 3.5 M HCl and stirred for 20 s before being allowed to stand for 16 h; after centrifugation, the supernatant was filtered and collected to determine the NAIP fraction. The residue was washed twice with 10 mL of deionized water, after which 20 mL of 1.0 M HCl was added. The mixture was stirred for 16 h and then centrifuged at 2000g for 15 min and filtered, and the extract was collected to determine AP fraction. To identify and quantify the P species and all the measured elements (Ca, K, Mg, Na, Mn, Fe, Al, Zn, Cu, Cr, and Pb), each sample was analyzed independently in triplicate using ICP-OES.
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RESULTS AND DISCUSSION The proximate and ultimate analyses of the PL and its HC have previously been investigated by Ghanim et al.14,15 To identify potential correlations between the HTC processing conditions and nutrient partitioning behavior, particularly for C
DOI: 10.1021/acssuschemeng.7b04768 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering Table 1. Phosphorous Speciation in PL and HC Samples MST mg g−1 ± SD ID PL 5−200 5−225 5−250 30−150 30−175 30−200 30−225 30−250 30−275 30−300 120−150 120−175 120−200 120−225 120−250 120−275 120−300 480−150 480−175 480−200 480−225 480−250 480−275 480−300 AA-9 AA-7 AA-4 SA-7 SA-4 SA-2
TP 14.63 15.79 28.24 37.27 10.92 14.97 20.74 39.38 56.35 69.16 62.35 20.70 19.81 31.35 51.12 48.32 51.68 57.53 17.31 17.41 35.51 48.75 50.29 54.25 62.42 62.10 60.81 47.13 66.45 46.87 17.44
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
IP 0.47 0.14 0.10 0.39 0.08 0.18 0.34 0.77 0.07 0.52 0.06 0.19 0.10 0.08 0.19 0.87 0.18 0.38 0.07 0.27 0.28 0.24 0.35 0.13 0.32 0.77 0.43 0.05 0.44 0.83 0.07
8.50 14.65 26.79 36.73 7.76 9.72 17.12 35.46 45.17 59.00 49.06 16.33 15.03 24.36 41.59 43.51 45.29 53.40 16.53 16.01 33.07 46.20 47.84 47.83 57.76 57.86 56.07 45.05 61.50 45.04 15.40
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
OP 0.42 0.02 0.27 1.08 1.38 0.02 0.50 0.89 1.49 0.54 0.65 0.26 0.04 0.32 0.37 0.48 0.17 0.41 0.16 0.03 0.72 0.38 0.67 0.15 0.11 1.57 0.05 0.50 0.33 1.13 0.33
0.89 0.11 0.15 0.19 0.25 0.10 0.13 0.25 0.25 0.39 0.44 0.20 0.25 0.33 0.16 0.15 0.71 0.61 0.12 0.16 0.15 0.24 1.19 1.63 0.39 0.12 1.01 0.60 1.29 1.28 0.24
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
AP 0.01 0.01 0.00 0.01 0.02 0.01 0.01 0.04 0.04 0.06 0.05 0.01 0.01 0.02 0.03 0.02 0.04 0.00 0.00 0.04 0.04 0.03 0.02 0.05 0.05 0.01 0.21 0.14 0.05 0.52 0.01
3.29 8.33 16.96 24.98 5.09 8.05 7.42 14.13 21.08 29.04 24.81 3.08 6.26 9.48 13.30 21.58 20.86 41.45 8.99 11.70 24.55 30.68 30.31 44.40 54.94 40.78 41.57 36.28 45.44 36.54 13.94
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
NAIP 0.20 0.12 0.11 0.16 0.18 0.33 1.09 0.27 1.14 2.09 0.26 0.09 0.34 0.72 0.94 4.42 5.65 0.34 0.38 0.10 0.64 0.90 0.20 0.06 0.12 1.38 0.38 0.15 0.36 0.91 0.25
3.30 2.91 3.18 4.26 2.10 1.81 0.91 4.76 6.17 7.39 3.36 5.18 1.63 1.79 5.99 3.87 2.57 2.83 3.04 0.83 3.79 5.90 0.97 1.78 1.43 6.51 7.15 3.25 6.25 1.13 0.08
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.10 0.06 0.04 0.00 0.04 0.02 0.01 0.05 0.07 0.77 0.12 0.17 0.16 0.01 0.21 0.02 0.14 0.06 0.02 0.01 0.07 0.11 0.06 0.05 0.04 0.08 0.09 0.02 0.13 0.13 0.02
WSP mg g−1 ± SD 7.85 7.17 2.64 1.59 4.48 6.59 1.47 1.67 1.08 1.99 0.55 7.80 3.39 1.53 1.57 0.68 0.36 0.51 6.59 1.40 1.31 1.56 0.12 0.44 0.41 1.67 2.54 1.59 2.02 0.71 2.79
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.04 0.01 0.01 0.03 0.02 0.03 0.05 0.00 0.01 0.04 0.01 0.03 0.01 0.00 0.03 0.00 0.01 0.00 0.00 0.02 0.01 0.00 0.00 0.00 0.00 0.01 0.11 0.00 0.01 0.00 0.03
WSP recovery % ± SD 100 46.27 17.21 8.23 50.63 63.89 7.72 8.37 4.59 6.88 2.11 80.67 18.63 8.98 7.09 2.88 1.38 1.71 60.79 8.03 6.74 6.73 0.50 1.63 1.27 5.89 8.21 4.88 6.25 2.85 13.22
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.00 0.06 0.09 0.14 0.22 0.34 0.29 0.00 0.07 0.12 0.05 0.34 0.04 0.02 0.13 0.05 0.03 0.01 0.15 0.11 0.16 0.09 0.00 0.06 0.00 0.06 0.22 0.00 0.02 0.01 0.16
10.9 to 69.2 mg g−1, generally increased with treatment temperature with the exception of the 30−300, 120−175, and 120−250 samples. A significant change in the TP concentration was observed at treatment temperatures ≥200 °C irrespective of residence time. The TP recovery measured around 60% at low treatment temperature, suggesting approximately 40% of the TP was extracted, after which it increased to 100% at higher treatment temperature. The WSP on the other hand generally showed a trend of decreasing with temperature. The WSP measured more than 50% in HCs produced at low treatment temperature but decreased sharply to around 2% in those HCs produced at the higher treatment temperature. Similar effects were observed by Ekpo et al.23 for HC obtained from HTC of PL at 250 °C for 1 h, where they concluded that the presence of multivalent metal ions (e.g., Al, Ca, Mg, and Fe) could be responsible for the formation of insoluble P. P immobilization was also observed in HC produced from HTC of manures and was attributed to the formation of insoluble phosphates during HTC treatment due to the presence of metal cations.7,25,32 The current observations were also consistent with those of Chen et al.37 and Silva et al.38 who produced HC from HTC of watermelon peel waste and from HTC of mixtures of vinasse and sugar cane bagasse under various conditions, respectively. They both reported that the P content retained in the HC increased sharply with an increase in treatment temperature. As can be observed from the results listed in Table 1 and illustrated in Figure 2, the IP represented the main P species
different. These differences are mainly due to the complex nature and heterogeneity of PL but also could be due to the different methods used for evaluation. Effect of Treatment Conditions on Nutrient Mobility. It is recognized that the main component of PL is organic matter which mostly degrades under HTC conditions.13,16,36 Ghanim et al.14,15 found that a considerable amount of the cellulose and hemicellulose in PL was easily hydrolyzed under the experimental conditions examined, leading to an increase in the ash content of HCs (conserved elements were enriched in the HC). The authors also reported a significant decrease in ash recovery, suggesting that a considerable amount of the inorganic material was removed. To clarify the correlation between the treatment conditions and nutrients species, the impact of the examined process conditions on different P species and other nutrients was investigated, and the results are listed in Tables 1 and 2 and are shown in Figures 2−5. The P recovery, which is defined as the product of the hydrochar yield times the ratio of the P in the HC to that of PL, was calculated. Furthermore, SEM/EDS analyses were carried out to verify the results and further investigate the HC composition and elemental distributions, and these results confirmed the effects of process conditions on nutrient recovery. Effect of Treatment Temperature. As shown in Figures 2 and 3, the treatment temperature had an obvious effect on the inorganic content of the HC samples. In the results presented in Table 1, it was found that the TP, which ranged from D
DOI: 10.1021/acssuschemeng.7b04768 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering Table 2. Total and Water-Soluble Nutrient Contents in PL and HC Samples total nutrient contentsa mg/g ± SD ID PL 5−200 5−225 5−250 30−150 30−175 30−200 30−225 30−250 30−275 30−300 120−150 120−175 120−200 120−225 120−250 120−275 120−300 480−150 480−175 480−200 480−225 480−250 480−275 480−300 AA-9 AA-7 AA-4 SA-7 SA-4 SA-2
Ca 27.02 47.96 50.64 60.55 33.51 45.01 79.98 78.48 99.34 116.45 117.75 43.59 74.15 82.64 98.60 87.45 92.53 93.23 45.19 62.98 64.52 88.89 89.31 96.26 117.51 122.24 105.50 94.25 105.11 86.53 62.77
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
K 0.85 1.19 0.10 0.21 1.50 2.31 0.76 1.17 0.56 5.46 1.14 1.01 1.17 1.16 0.58 0.00 0.46 4.17 0.40 0.43 2.47 1.33 0.22 0.19 2.41 1.43 2.26 0.27 0.08 2.91 0.55
38.61 27.44 20.27 8.56 9.30 15.87 0.22 0.57 1.96 0.91 1.76 44.14 11.87 1.63 1.74 1.47 1.44 0.93 22.53 1.28 0.46 0.61 1.21 1.00 0.89 4.57 1.22 5.16 6.30 7.27 8.50
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
water-soluble nutrient contentsb mg/g ± SD
Mg 1.10 0.00 0.03 0.02 0.22 0.27 0.07 0.01 0.01 0.03 0.01 0.12 0.09 0.05 0.01 0.00 0.01 0.00 0.08 0.00 0.05 0.01 0.04 0.03 0.01 0.05 0.02 0.02 0.02 0.26 0.10
8.06 7.79 11.28 18.22 6.62 8.69 6.01 14.75 28.26 35.14 34.24 12.02 8.21 11.05 23.31 25.31 28.03 29.73 9.67 5.68 14.61 23.59 19.73 28.75 33.75 29.32 29.00 13.80 31.72 22.42 15.19
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
Na 0.31 0.13 0.16 0.15 0.00 0.02 0.01 0.09 0.06 0.52 0.11 0.07 0.18 0.11 0.39 0.67 0.14 0.01 0.08 0.03 0.08 0.01 0.15 0.22 0.85 0.32 0.32 0.01 0.13 0.51 0.10
3.50 3.02 2.34 1.24 0.00 0.53 0.00 0.07 1.02 0.43 0.88 4.84 1.35 0.41 1.14 1.34 0.63 0.77 2.34 0.20 0.31 0.45 0.51 0.81 1.00 1.00 0.87 0.81 1.20 0.95 0.84
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
Ca 0.31 0.01 0.02 0.00 0.00 0.08 0.00 0.00 0.01 0.01 0.00 0.06 0.01 0.01 0.02 0.01 0.00 0.00 0.01 0.00 0.00 0.01 0.01 0.01 0.00 0.00 0.01 0.05 0.01 0.02 0.04
1.03 1.27 0.32 0.16 0.83 0.76 1.11 0.33 0.20 0.06 0.18 0.87 1.20 0.63 0.15 0.21 0.42 0.13 0.67 0.94 0.28 0.04 0.70 0.22 0.15 0.16 0.03 1.02 0.06 7.28 48.21
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
K 0.05 0.02 0.00 0.00 0.01 0.01 0.00 0.01 0.01 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.00 0.01 0.02 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.05 0.05
30.95 22.83 16.32 7.30 8.79 15.24 0.14 0.34 0.62 0.36 0.24 25.07 7.04 0.50 0.65 0.35 0.33 0.16 16.62 0.69 0.22 0.25 0.13 0.21 0.13 0.65 2.92 3.57 4.26 4.24 6.26
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
Mg 0.53 0.03 0.07 0.01 0.02 0.04 0.00 0.00 0.01 0.01 0.00 0.08 0.02 0.01 0.01 0.00 0.00 0.00 0.04 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.01 0.01 0.00 0.19
2.94 3.27 2.44 1.40 2.34 3.81 0.86 1.75 1.32 2.50 0.56 3.92 1.98 1.41 1.77 0.74 0.58 0.74 3.43 0.82 1.70 1.89 0.11 0.64 0.63 2.15 2.51 1.37 2.31 0.74 0.90
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
Na 0.15 0.01 0.01 0.01 0.00 0.01 0.02 0.01 0.01 0.05 0.00 0.01 0.02 0.02 0.04 0.00 0.01 0.01 0.00 0.01 0.00 0.01 0.00 0.00 0.00 0.01 0.05 0.00 0.00 0.01 0.02
4.43 3.20 2.24 1.03 1.16 2.03 0.05 0.10 0.24 0.16 0.09 3.38 0.96 0.14 0.35 0.14 0.09 0.09 2.19 0.14 0.10 0.10 0.03 0.11 0.11 0.23 0.48 0.46 0.65 0.42 0.79
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.10 0.00 0.01 0.00 0.00 0.03 0.00 0.00 0.01 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.01 0.00 0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.03
a
Measured using SMT protocol. bMeasured using aqueous extraction procedure.
at higher treatment temperature. These observations are generalizations with some exceptions, which could be attributed to a lack of measurement. Ekpo et al.23 made a similar observation and reported that most of the Ca Mg, P, Fe, and Al were concentrated in the HC. Silva et al.38 observed an increase in Ca, Mg, Fe, Mn, and Zn concentration with treatment temperature with no noticeable change in Al and Cu concentrations. Tu et al.39 who prepared HC from HTC of PL at 120 and 200 °C for 30 and 120 min, as well as Chen et al.37 and Sun et al.40 have reported a reduction in K concentration with treatment temperature and attributed this to the solubility of K compounds. From the results (Supporting Information, Table S6), the HC produced had a molar ratio of Ca/P > 1; on the other hand, the P exhibited a high molar ratio compared with that of K and Mg. As reported by Rahman et al.,41 many factors such as chemical composition (molar ratio of Ca/P, Mg/P, and Mg/Ca), pH, and temperature can influence the quantity and type of precipitates. Therefore, the possible precipitates containing P, Mg, and Ca that can be recovered under the conditions examined include struvite-Mg, Mg phosphate, Ca phosphate, Ca carbonate, and apatite.41 Taking into account the Mg/P (
