Functionalization of kaolinite for removal of phosphate from urban sewage

The PO43− widespread in urban sewages promotes eutrophication of water sources, with harmful effects to natural life and endanger human health. The removal of PO43− from urban sewage requires treatment at tertiary level, with high costs and low efficiency in most cases. Thus, a functionalization method for surface modification of kaolinite was proposed to improve the removal of PO43− from urban sewage. The kaolinite commercial did not remove PO43- from aqueous solution. However, the functionalized kaolinite (FK) was efficient, with a maximum removal capacity of 8.4 ± 0.1 mg PO43−/L, within less than 1 min of reaction. The removal of PO43- is associated with precipitation of pyromorphite, a mineral with low solubility (Ksp < 10−79,6). Finally, real urban sewage samples (raw and treated) were also tested for removal of PO43- using FK, confirming its effectiveness. The central aspects of this development are:• Functionalized kaolinite (FK), with Pb(II), for removal of PO43− from urban sewage was studied.• The FK was efficient for removal of up to 8.4 mg PO43−/L from aqueous solution, within a short reaction time.• The precipitation of pyromorphite was the mechanism responsible for removal of PO43- and FK efficiency have been confirmed for real urban sewage samples.


a b s t r a c t
The PO 4 3 − widespread in urban sewages promotes eutrophication of water sources, with harmful effects to natural life and endanger human health. The removal of PO 4 3 − from urban sewage requires treatment at tertiary level, with high costs and low efficiency in most cases. Thus, a functionalization method for surface modification of kaolinite was proposed to improve the removal of PO 4 3 − from urban sewage. The kaolinite commercial did not remove PO 4 3from aqueous solution. However, the functionalized kaolinite (FK) was efficient, with a maximum removal capacity of 8.4 ± 0.1 mg PO 4 3 − /L, within less than 1 min of reaction. The removal of PO 4 3is associated with precipitation of pyromorphite, a mineral with low solubility ( K sp < 10 −79,6 ). Finally, real urban sewage samples (raw and treated) were also tested for removal of PO 4 3using FK, confirming its effectiveness. The central aspects of this development are: • Functionalized kaolinite (FK), with Pb(II), for removal of PO 4 3 − from urban sewage was studied.
• The FK was efficient for removal of up to 8.4 mg PO 4 3 − /L from aqueous solution, within a short reaction time.
• The precipitation of pyromorphite was the mechanism responsible for removal of PO 4 3and FK efficiency have been confirmed for real urban sewage samples.

Background
The PO 4 3 − present in urban sewage promotes eutrophication in the water bodies [1] . This anion is difficult to remove during the treatment of urban sewage, requiring treatment at the tertiary level [2] . The most used method for the removal of PO 4 3 − from urban sewage is the chemical precipitation, involving the addition of bivalent or trivalent metal salts [3][4][5] . Recently, studies have shown the precipitation of pyromorphite (5Pb 2 + + 3PO 4 3 − + H 2 O → Pb 5 (PO 4 ) 3 (OH) (pyromorphite) + H + ) in natural surface waters due to presence of PO 4 3 − and Pb(II), reducing the concentration of dissolved Pb(II) [ 6 , 7 ]. The pyromorphite has a low solubility constant ( K sp < 10 −79,6 ), preventing that the Pb(II) returns to the environment as a dissolved cation [8][9][10] .
Kaolinite [Al 2 Si 2 O 5 (OH) 4 ] has its negatively-charged surface, becoming this mineral an important adsorbent for cationic ions [11] . The functionalization of kaolinite with acid treatment [12] and bivalent trace elements can promote the removal of anionic molecules, such as PO 4 3 − , present in the urban sewage [13][14][15] . Based on the functionalization of commercial kaolinite (CK) with bivalent trace elements, the functionalized kaolinite (FK) with Pb(II) was produced. The efficiency for removal of PO 4 3from aqueous solution using FK was studied and compared with CK. Furthermore, the reaction time and maximum removal capacity of PO 4 3was determined using FK. Finally, the FK was applied in real urban sewage samples, attesting its effectiveness.

Functionalization of commercial kaolinite
The CK (Sigma-Aldrich®, CAS Number 1318-74-7) was used in this study. The following procedures have been applied for the functionalization: 1 -About 1.0 g of CK was placed in a beaker; 2 -10 mL of aqueous solution with Pb(II) initial concentration of 40 mg/L was added; 3 -The beaker was agitated (digital shaker Biothec -model BT 645) for 24 h at 145 rpm; 4 -The solution was centrifuged (centrifuge Excelsa II® -model 206-BL) for 25 min at 30 0 0 rpm; 5 -The FK was separated and washed three times, using ultrapure water (Milli-Q® system -model IQ 70 0 0) with electrical conductivity lower than 0.02 μS/cm; 6 -Finally, the FK was dried for 12 h at 40 °C.

Method validation
For validation purposes, 1.0 g of each sample (CK and FK) was mixed with 10 mL of aqueous solution containing PO 4 3 − in the initial concentration ( C 0 ) of 1 mg/L at pH 6 [9] . The suspension was shaken for 24 h at 145 rpm, and then centrifuged at 30 0 0 rpm for 25 min. The supernatant was separated and the PO 4 3concentration remaining in solution ( C e ) was measured using a Hach DR-2800 spectrophotometer, with a detection limit of 0.1 mg/L. The removal efficiency of PO 4 3-( %A -in percentage) was determined according to the Eq. 1 . The experimental procedures were carried out in triplicate.  The results are presented in Table 1 . The removal efficiencies of PO 4 3 − were 0 and 100% using CK and FK, respectively. These results evidenced the functionalization plays a crucial role on the removal of PO 4 3 − . The precipitation of pyromorphite was the main mechanism associated with PO 4 3removal using FK with Pb(II), as shown in Fig. 1 .

Reaction time and maximum removal capacity of PO 4 3using functionalized kaolinite
The reaction time for removal of PO 4 3using KF has been investigated, according the following procedures carried out in triplicate. The FK (1.0 g) was mixed in 10 mL of an aqueous solution with C 0 of 1 mg PO 4 3 − /L at pH 6 [9] . The suspension was shaken at 145 rpm, with samples taken after 1, 5, 15, 30 and 60 min. The solution was centrifuged at 30 0 0 rpm for 25 min, with the supernatant separated and the C e determined. The experiments have shown no residuals of PO 4 3after 1 min of reaction time ( Table 2 ), showing a fast reaction time for removal of PO 4 3 − associated to the mineral pyromorphite precipitation.
The maximum removal capacity of PO 4 3 − using FK was also determined (in triplicate). The samples with 1.0 g: 10 mL of an aqueous solution with C 0 of 1 mg/L were stirred at 145 rpm for 5 min at pH 6 [9] , with C 0 of 1, 2, 3, 4, 7 and 9 mg/L. The solutions were centrifuged for 25 min at 30 0 0 rpm and the C e determined in the supernatants. The maximum removal capacity of PO 4 3 − using FK was 8.4 ± 0.1 mg/L ( Table 3 ) or 8.4 mg/g, indicating an efficiency of 93.3% for removal of PO 4 3from aqueous solutions with C 0 of 9 mg/L. The value of 8.4 mg/g is higher than the removal capacities obtained for natural or functionalized kaolinite, i.e., CK used in this study ( < 0.1 mg/g), kaolinite from Linthipe (ca. 0.15 mg/g) [12] , modified kaolinite with FeCl 3 (1.31 mg/g) [13] and modified kaolinite with seawater in different temperatures (4.07 mg/g at 600 °C) [15] .
Trace levels of residual Pb(II) in the treated effluent can pose a serious environmental risk for aquatic systems due to its toxicity. Thus, the concentration of Pb(II) were also determined in the

Removal of PO 4 3using functionalized kaolinite in real urban sewage samples
Three samples of raw and treated urban sewage were collected in a wastewater treatment plant (WWTP) located in Rio Claro, São Paulo State, Brazil. These samples were stored in labeled amber container at 4 °C and transported immediately to the laboratory, where they were filtered, using 0.45 μm MF-Millipore® membrane filter, and the C 0 of PO 4 3 − and Pb(II) measured ( Table 4 ). In order to verify the real removal efficiency of PO 4 3 − from urban sewage (raw and treated), 10 mL of each filtered sample were mixed with 1.0 g of FK. The solutions were shaken at 145 rpm for 5 min at pH 6 [9] , centrifuged at 30 0 0 rpm for 25 min, and then the C e of PO 4 3 − and Pb(II) were determined in the supernatants ( Table 4 ).
The C 0 averages of PO 4 3were 6.1 ± 0.1 e 3.8 ± 0.1 mg/L, respectively, for raw and treated urban sewage. After the use of FK, C e averages of PO 4 3were lower than 0.1 mg/L. In addition, the C 0 and C e averages of Pb(II) were always lower than the detection limit of 0.006 mg/L. These results show the efficiency during the use of FK for removal of PO 4 3 − from urban sewage in real samples collected in a WWTP.

Conclusions
A method for functionalization of kaolinite for removal of PO 4 3 − from urban sewage was studied. The functionalized kaolinite (FK) with Pb(II) have shown a promising alternative for removal of PO 4 3 − in aqueous solution, with maximum removal capacity of 8.4 mg/L, within a reaction time lower than 1 min. The precipitation of PO 4 3is associated with pyromorphite, a mineral with low solubility ( K sp < 10 −79,6 ). Finally, real urban sewage samples (raw and treated) were also tested for removal of PO 4 3using KF, confirming its effectiveness for removal of PO 4 3 − from urban sewage with C 0 4 mg/L.