Sodium hydroxide (NaOH), acetic acid (CH₃COOH) and magnesium chloride (MgCl₂∙6H₂O) were obtained from the Kanto Chemical Co. Inc (Tokyo/Japan). All chemicals and reagents were of analytical grade used with purification by dissolved of distilled water.

Activated sludge is obtained from wastewater treatment plant, Miyoshi city, Hiroshima Prefecture. We collected which have already to become carbonized by factory. Coffee husk was collected from previous of our studied [18]. The dried sample was crushed and passed through 4.75 m/m sieve at 60˚C for 24 h in oven. The muffle furnace (model F0100 Yamato, Japan) is used to make biochar with temperature 600˚C for 2 h. The biochar passes through a sieved < 1 mm and washed with distilled water, oven dry at 60˚C for 24 h and sealed in a beaker for further studies. The characteristics of biochar were presented in Table 1.

Activated sludge charcoal used for phosphate extraction. The concentration was used NaOH with concentration from 0.1, 0.2, 0.3, 0.4 and 0.5 M for 24 hours. The samples were carried out with 1 gram in 50 mL of solution. While for potassium extraction used coffee husk biochar, we used CH₃COOH with concentration from 1, 2, 3, 4 and 5 M for 30 minutes. The sample was carried out with 0.25 gram in 50 mL of solution.

The best results of phosphate and potassium extraction were used. 50 mL of

1000 ppm magnesium solution was added to fixed amount of obtained solution. Phosphate and potassium were added 20 and 50 mL, respectively. The detailed of procedure in Figure 1 and Figure 2.

The concentrations of PO 4 3 − were determined by standard method (Japan Industrial Standard method JIS KO 102). The concentration of potassium was measured using atomic absorption spectrophotometer (AA-6300, Shimadzu Kyoto, Japan). Cation exchange capacity (CEC) was extracted 1 M NH₄OAc pH 7. Morphology and chemical composition of crystal was measured by Scanning Electron Microscopy (SEM) with Energy Dispersive X-Ray Analysis (EDX) (JED-2300) (Shimadzu, Kyoto, Japan).

1) First precipitation

2) Cation removal for eliminated of aluminium to get struvite-K precipitates

We examined cation removal especially for removed Al to get pure struvite-K by dissolved 0.5 M HNO₃ for 1 hour with ratio of 1/100 of solid sample/liquid. 1000 ppm of 50 mL of the potassium and magnesium solution from KH₂PO₄ and MgCl₂∙6H₂O, respectively. The solution was adjusted to pH 11 by NaOH. The detailed procedure is in Figure 2.

Phosphate recovery from sewade sludge with alkaline conditions has been studied in some authors [19]. Alkaline extraction is a good alternative since it gives a phosphorus product with a lower metal contamination. Most metals are most insoluble in alkaline than in acids, and extraction with alkaline gives a phosphorus product with a lower contamination of metal [20]. However, the process of

extraction, aluminium was released which simultaneously with P. To confirm the release of aluminium, we examined extraction of activated sludge charcoal of 50 mL with pH of 12.8. This pH value is considering low precipitation. However, we must adjust to pH 11 by HCL [21].

To compete aluminium, we added MgCl₂∙6H₂O as magnesium source to adjusted molar Mg: P of 1:1 with formula in Equation (1), since this value is considering favorable for precipitation in many scientists. Stirring for 2 hours and aging for 24 hours then filtered. The solid precipitate was obtained then oven dried 60˚C. The results were obtained in Figure 3 and composition of precipitation as shown in Table 2.

2AlPO 4 + 3MgCl 2 → Mg 3 ( PO 4 ) 2 + 2AlCl 3 (1)

Table 3 presents the results of phosphate recovery under different concentration of NaOH. It is found that the phosphate recovery increased, when concentration of NaOH increased from 0.1 M to 0.3 M. Thereafter, the phosphate recovery decreases from 0.3 M to 0.4 M and slightly increased from 0.4 M to 0.5 M. This is indicated that around this concentration phosphate recovery from activated sludge charcoal was effective. This is agreement with [22] that recovery of sludge in Karlskoga with sodium hydroxide reaches 90% of the phosphorus content.

Potassium is a chemical element with the symbol K and atomic number 19. Potassium reacts rapidly with oxygen. Carboxylic acid is an organic acid that contains a carboxyl group and contains oxygen with formula (C(=O)OH). The present study, we used acetic acid (AA). Acetic acid is the second simplest carboxylic acid (after formic acid). In the other hand, acetic acid is widely used bulk chemical and it can be used as green solvent that non-toxic in environment [23].

Acetic acid is acidic character because release of the proton (H⁺). Furthermore, acetic acid is in the carboxyl group (-COOH) can separate from the molecule by ionization in Equation (2) and reacts with cation such as potassium with formula in Equation (3).

CH 3 COOH → CH 3 CO 2 − + H + (2)

K 2 O + CH 3 COOH → CH 3 COOHK + H 2 O (3)

Table 4 presents the recovery of potassium under different concentration of acetic acid (AA). It is found that the potassium recovery fluctuating in all concentrations. The lower results in 4 M CH₃COOH with concentration of 26 mg/L while the higher in 5 M CH₃COOH with concentration of 1891 mg/L. This is indicated that higher concentration of acetic acid can easily react with potassium.

The present study, we examined the reaction conditions for the first precipitation. Experimental conditions we conducted in glass and stirring for 2 hours. The solution is from 50 mL of 1000 ppm magnesium from MgCl₂∙6H₂O. The

phosphate solution is added 20 mL from 0.3 M NaOH extractants as shown in Table 3. While for potassium solution is added 50 mL from 5 M CH₃COOH as shown in Table 4. However, the aluminium was released simultaneously with P from activated sludge charcoal which has described in above as shown in Figure 3. While for this precipitation we added potassium to make composition of struvite-K (MgKPO₄∙6H₂O) as shown in Figure 4.

The results show that the aluminium content is 0.85 wt% (Table 5) which lower than Figure 3 is 5.37% (EDX results from activated sludge charcoal). This is indicated that potassium can compete with aluminium in precipitation with precipitate of Al: P of 1, Mg: P of 3 and K: P of 0.5 (in mol/l). Calculated from wt% to mol/l in Equation (4).

wt%/Fw (4)

Morphology of precipitation as shown in Figure 5. Seen that needle-like mixed with tetrahedral structure, indicated that aluminium reacts with phosphate to become aluminium phosphate precipitates [24]. However, to get pure of struvite-K we must examine or release of aluminium in solid precipitation. On the other hand, benefit of aluminium is can be used as coagulate of organic matters which occurs of eutrophication [25]. Moreover, if aluminium can directly use in soil as fertilizer, it may have negative effect on plant growth [26].

After the first precipitation, we examined cation removal especially to removed aluminium from precipitation product to get struvite-K by dissolved 0.5 M HNO₃ with reacts with aluminium with formula in Equation (5):

Al 2 O 3 + 6HNO 3 → 2Al ( NO 3 ) 3 + 3H 2 O (5)

KH₂PO₄ and MgCl₂∙6H₂O as potassium and magnesium sources, respectively as solution of struvite-K precipitates. The solution was adjusted to pH 11 by NaOH. The results show in Figure 6 with chemical composition in Table 6.

Based on Figure 6, the aluminium was successfully removed from solid precipitation. This is indicated that HNO₃ was effectively used to release of cation especially aluminium with precipitate of K: P of 0.5 and Mg: P of 0.8. Furthermore, struvite-K precipitates during processing which fall to become sediment as shown in Figure 7. And if have dried, struvite-K precipitates can be seen as crystal through SEM images as shown in Figure 8. The shape is needle-like which have reported in some experimental works [27] [28].