Synthesis of Struvite Crystal (MgNH
4PO
4.6H
2O) from Laundry Waste to Consider its Potential as a Plant Fertilizer: Stirring and
Processing Temperature Effect
Dyah Suci Perwitasari,1 Nur Aini Fauziyah,1* and Pardi Sampe Tola,2
1Department of Chemical Engineering, Faculty of Engineering, Universitas Pembangunan Nasional “Veteran”
Jawa Timur (UPN), Gunung Anyar, Surabaya 60294, Indonesia
2Department of Food Technology, Faculty of Engineering, Universitas Pembangunan Nasional “Veteran” Jawa Timur (UPN), Gunung Anyar, Surabaya 60294, Indonesia
*Corresponding email: [email protected] Email author 1: [email protected]
Email author 2: [email protected] Received 12 July 2022; Accepted 18 November 2022
ABSTRACT
This paper aimed to process phosphate from laundry waste to be struvite crystal by precipitation and crystallization methods. In general, phosphates are difficult to remove by conventional treatment technologies. Therefore, precipitation and crystallization methods can be an alternative choice for phosphate recovery. Precipitation and crystallization methods by adding KOH can serve to remove dissolved phosphate content in wastewater, as well as convert it into a solid form that can be reused as industrial raw materials. The research variables used were stirring temperature (25, 30, and 35 C) and stirring speed (100, 125, 150, and 200 rpm). By using X-ray Fluorescence (XRF) and Scanning Electron Microscope (SEM-EDX), the analysis showed that the highest percentage of phosphate removal was at a stirring speed of 100 rpm at 30°C, which was 47.5%. X-ray Diffraction (XRD) was also carried out on several samples and it was confirmed that the dominant crystal phase formed was a struvite (MgNH4PO4.6H2O) for all samples, and secondary phase magnesium phosphate hydrate (Mg3(PO4)2.8H2O was found in a stirring speed of 100 rpm at 30°C. The morphology of the struvite crystals formed resembles irregular flakes by using SEM.
Keywords: laundry waste, struvite, phosphate, precipitation and crystallization method.
INTRODUCTION
Household and industrial waste contains a lot of phosphate [1], [2], one of which is generated from laundry activities. Laundry services aim to ease household chores and create jobs. Laundry services in addition to having advantages, there are disadvantages, where the waste generated can pollute if discharged directly into water bodies.
Laundry waste is generated from detergents that have active ingredients. Most detergents on the market include phosphate and LAS (Linear Alkyl Sulfonate), a surfactant. If the laundry's remaining detergent water is drained away, the extra phosphate that is left on the surface causes water contamination (eutrophication phenomenon). In addition to carbon (C) and nitrogen (N), high amounts of phosphate in water are the primary nutrient in the eutrophication process [3].
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Laundry waste has a phosphate content that exceeds the quality standard for liquid waste according to (KEP 51-/MENLH/10/1995), the maximum limit is 3 mg/L. If phosphate is disposed of without prior treatment, it will harm the environment. Too much phosphate contained in water will accelerate the growth of algae in excess (eutrophication) [4], thus closing the water surface, which results in inhibition of sunlight penetrating and reducing oxygen levels below the water surface [5], [6].
Laundry waste pollution causes many environmental problems so that researchers treat laundry waste first. Laundry waste treatment is carried out to reduce phosphate and ammonium levels. Previous researchers used a variety of different methods including the moving bed biofilm reactor method [7], [8], plant biosystems [9], [10], adsorbents as filtration media [11], [12], electrocoagulation processes [13], [14], and biosand filters to degrade laundry waste [15], [16].
The latest laundry waste treatment which is reused as struvite fertilizer (MgNH4PO4.6H2O) has a high selling price. Struvite fertilizer which has low solubility and is slow to lose nutrients and the quality of the fertilizer (slow release fertilizer) is good for plants and environmentally friendly [17]. The method used was the precipitation and crystallization process, which was a type of reaction in a liquid that produces a precipitate. This method has the advantage that it is very simple using a magnetic stirrer.
Based on these advantages, in this research, struvite will be synthesized from laundry waste using crystallization and precipitation methods. The variables regulated in this study were the mixing temperature variable and the stirring speed, so as to produce struvite crystal deposits. The resulting product will be characterized by XRF, XRD, and SEM-EDX. The novelty of this research is the application of the stiring method and low temperatures in the synthesis of struvite (> 100 C).
EXPERIMENT
Chemicals and instrumentation
In this study used a source of phosphate from laundry waste, as well as KOH 1N to adjust the pH of the laundry waste solution that was formed. While the characterization needed was XRF to determine the elemental content in laundry waste, XRD Philips X’Pert MPD diffractometer (Cu-Kα radiation: = 0.154056 nm at 40 kV and 30 mA) to confirm whether or not struvite has been formed, and SEM (Carl Zeiss) instrument with Energy-dispersive X-ray (EDX) to determine the morphology and elements in the formed struvite.
Procedure reaction
In this experiment, the preparation of the solution was started by dissolving MgCl2 and NH4H2PO4 crystals with a molarity ratio of 1: 1: 1, respectively for Mg2+, NH4+ and PO43-.The pH of the initial solution was ±9.0 with the addition of KOH. The reaction time and pH of struvite crystallization were evaluated in this experiment and carried out at a temperature of 25, 30, and 35 C and stirring cycles of 100, 125, 150, and 200 rpm.
The experiment was carried out by mixing the two solutions (MgCl2 and NH4H2PO4) in a 1000 ml glass beaker. The solution in the glass beaker was stirred in various speed to make the homogenous solution. The crystallization process continues to be observed by checking to ensure that the pH remains 9. The results in the beaker were filtered using filter paper to separate the filtrate from the precipitate. Afterward, the precipitation was dried and
characterized the struvite crystals with SEM-EDX and the P2O5 content in struvite using XRF and XRD.
RESULT AND DISCUSSION
First, the analysis (phosphate, amonium, detergen anionic and pH) of precondition from laundry waste was done and the results was present in Table 1. From the results of the laundry waste (raw material) for making struvite fertilizer, laundry waste has a phosphate content that exceeds the quality standard. According to (KEP 51-/MENLH/10/1995), the maximum limit of phosphate content in wastewater is 3 mg/L. Phosphate that is disposed of without prior treatment can harm the environment. Therefore, it is necessary to treat laundry waste first.
Table 1. Initial content of laundry waste
No Parameter Satuan
1 phosphate 23,7 mg/l
2 Amonium 5,3 mg/l
3 Detergen anionic 8,15 mg/l
4 pH 9,3
Based on the results of the initial analysis shown in Table 1, the synthesis of struvite was carried out according to the method described previously and the results of diphosphorus pentaoxide content showed in Table 2 as follows.
Table 2. Weight percentage of diphosphorus pentaoxide (P2O5) using XRF Stirring speed
(rpm)
Mixing Temperature (C) P2O5
(mole %)
The quality standard
100 25 46.7 A
30 47.5 A
35 47.4 A
125 25 46.3 A
30 45.4 A
35 45.7 A
150 25 46 A
30 43.3 A
35 43.9 A
200 25 45 A
30 44.1 A
35 44.5 A
Based on Table 2 above, with a stirring speed of 100 rpm with different temperatures, the levels of P2O5 at a temperature of 25C to 30C increased and decreased at a temperature of 35C. In this study, the increase occurred due to heating rate which allowed decomposition
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and Garrido [18], by experimental kinetics of reversible reaction of crystallization of struvite using temperatures at 25, 30, and 35 C respectively, it was shown that at high temperatures the dissolving of struvite was preferable. Therefore, struvite is more soluble at 30C than 25C.
But in another study observed that the solubility increased with increasing temperature up to 30C and then decreased. According to the theory of Bhuiyan [19] with the statement that struvie crystallization has increased to a temperature of 30C and decreased at a temperature of 35C. In this study, the highest P2O5 level was at 30C. Based on the SNI 02-0086-2005, The P2O5 content greatly determines the quality of the fertilizer. The fertilizer will be of grade A if it contains at least 40% mole (Table 2).
While the effect of stirring can be seen clearly from Table 2. When compared at the same processing temperature with different stirring speeds, it was found that P2O5 levels decreased from a stirring speed of 125, 150, and 200 rpm. This decrease is in accordance with previous studies which said that the greater the stirring speed, the lower the induction time of nucleation.
The decrease occurs because the stability of the struvite crystals at too high agitation can decrease and cause the crystals to break [20]. In this study, the highest level of P2O5 was at a stirring speed of 100 rpm.
Figure 1. Legend should be written in bold and place below the figure or scheme.
The successful formation of the struvite phase was confirmed by XRD. Figure 1 shows a representative sample of struvite with a stirring speed of 100 rpm. It can be seen that the struvite was completely formed at 25 and 35C at 100 rpm. Both of these samples showed that pure of the struvite phase [PDF No#00-071-2089] had been formed. but the sample of 30C at 100 rpm showed the presence of a secondary phase, namely Magnesium Phosphate Hydrate [PDF No#00-033-0878]. The possibility of the formation of this phase is an excess of MgCl2
and NH4H2PO4 which does not react completely to form struvite crystals. Based on Rietveld
analysis, it was found that 8.7% was Magnesium Phosphate Hydrate phase. However, the nature of Magnesium Phosphate Hydrate phase on fertilizer quality did not have a bad impact, because it was also a source of nutrients for plants. If the content of this secondary phase is lost, the quality of the fertilizer will be better [17], [21].
The morphological characterization of struvite using SEM-EDX aims to determine the shape, size and particles that make up struvite. Based on Perwitasari et al. [22], irregular flake- shaped of struvite crystals was about 20 m. The characteristics of struvite crystals are influenced by the content contained in the struvite crystals. The smaller struvite crystal size has a larger total surface area so that it will accelerate the solubility of a substance and the process of absorption of fertilizer nutrients can take place more quickly. Considering that struvite is a slow-release fertilizer, the larger crystal size causes nutrients in the fertilizer to be absorbed more slowly by plants [23]. In the SEM analysis with a temperature of 25C and 30C at 100 rpm have irregular crystal forms of about 15-20 m (Figure 2).
Figure 2. The morphological characterization of struvite using SEM a. 25C and b. 30C at 100 rpm
The EDX spectrum shows Mg, P, and O peaks which are the main elements of struvite crystals (Table 3). C-Organic content is an important factor determining the quality of mineral soil. The higher the total C-Organic content, the better the mineral soil quality. K element greatly determines the quantity and quality of plant yields because this nutrient plays a role in the process and translocation of photosynthetic products, protein synthesis, and increases plant resistance to biotic (pest/disease) and abiotic stresses (water shortage and iron poisoning).
Table 3. Atomic percentage of element from struvite samples (25C and 30C at 100 rpm) using SEM-EDX
Element 25C
(atomic %)
30C (atomic %)
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C 25.84 38.05
N 5.09 5.11
O 48.4 44.38
Na 0.54 0.27
Mg 9.23 5.76
Al 0.44 0.72
Si 0.89 0.51
P 7.36 3.85
S 0.62 0.25
Cl 0.29 0.22
K 0.69 0.54
Ca 0.61 0.35
CONCLUSION
The results of the study of taking phosphate from laundry wastewater with the struvite crystallization process obtained the highest P2O5 levels at a stirring speed of 100 rpm and a temperature of 30C at around 47.5% cointained two phases, i.e., struvite as main phase and Magnesium Phosphate Hydrate as secondary phase. The morphology of the irregular flake crystals is a typical form of struvite crystals with a size of 15-20 μm.
ACKNOWLEDGMENT
The authors would like to thank the LPPM UPN “Veteran” of East Java who has financed the implementation of the internal research program of the Advanced Basic Research Scheme (RISLA) in accordance with the contract number: SPP/37/ UN.63.8 /LT/ IV/2022
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