TiO 2 P-25 anatase rapid precipitation from water by use of struvite formation I. Blumberg a , J. Starosvetsky a , D. Bilanovic b , R. Armon a, * ,1 a Faculty of Civil & Environmental Engineering, Division of Environmental, Water & Agricultural Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel b Center for Environmental, Earth, and Space Studies, Bemidji State University, Bemidji, MN 56601, USA article info Article history: Received 19 September 2008 Accepted 13 March 2009 Available online 31 March 2009 Keywords: Titanium dioxide (TiO 2 P-25) Powder Struvite Precipitation Water Method abstract UV 360nm irradiation of TiO 2 P-25 nanoparticle in water suspension is used for photocatalytic mineraliza- tion of pollutants or inactivation of microorganisms. Removal of TiO 2 particles from large volumes of water following photocatalytic process is problematic due to their nano-size. So far no chemical methods are available for TiO 2 rapid precipitation while filtration or centrifugation is not feasible because of high cost and limited performance. In the present study TiO 2 was rapidly precipitated from water suspension by formation of the mineral struvite. Addition of Mg, P, and NH þ 4 at stochiometric rates of 1:1:1 at pH > 8.3 resulted in TiO 2 entrapment into struvite formed flocs and rapid precipitation. Struvite sludge formed and precipitated was observed under HSEM revealing TiO 2 entrapment into struvite mineral. Ó 2009 Elsevier Inc. All rights reserved. 1. Introduction TiO 2 in its anatase form is the most common photocatalyst used experimentally for degradation of xenobiotic organics and inacti- vation of pathogenic microorganisms [1–4]. The frequent applied practice of photocatalytic process in water systems is based on addition of TiO 2 anatase powder (optimally 1 g L 1 ) to an experi- mental water volume followed by direct or intermittent irradiation of the suspension with black UV (360 nm) for certain time intervals [5]. The main drawback of this advanced oxidation process is the residual TiO 2 colloidal particles that have to be removed from water before supply. Due to colloidal size of TiO 2 , natural precipi- tation within an acceptable time interval is not possible. Practical solutions can be based on physical removal through centrifugation or membrane filtration methods; however, these approaches are limited to small volumes or subject to rapid clogging, respectively. To our knowledge there are no chemical methods to remove TiO 2 anatase colloidal particles from suspensions. Independently, a common problem observed with anaerobic digested sludge lines is formation of a crystalline material along the pipelines identified as the mineral struvite or MAP (magnesium, phosphate and ammo- nium) [6,7]. Struvite is a white crystalline substance comprising magnesium, ammonium and phosphorous in equal molar concen- trations (MgNH 4 PO 4  6H 2 O) that forms crystals with a distinctive orthorhombic structure. The general reaction is shown below: Mg 2þ þ NH þ 4 þ PO 3 4 þ 6H 2 O ) MgNH 4 PO 4  6ðH 2 OÞ# When the basic components of the mineral are present at high con- centrations in any solution at pH > 7, they will precipitate instantly following two stages: nucleation and growth. The growth of struvite crystal will continue until equilibrium is reached [8]. If struvite con- stituents are supplied continuously the crystal growth may con- tinue perpetually. The main idea behind the present study was to use struvite formation to co-precipitate TiO 2 colloidal suspension. The results and its possible utilization are discussed. 2. Experimental Struvite constituents were dissolved in double distilled water. MgCl 2  6H 2 O (Merck, Germany), NH 4 Cl (Merck) and Na 2 H- PO 4  2H 2 O (RDH Laborchemikalen, Germany) were dissolved to obtain stock solutions of 0.2 M. pH of stock solutions was adjusted with HCl or NaOH (1 N) to reach a value of 10 for all solutions. TiO 2 P-25 (Degussa AG, Germany) was suspended in double dis- tilled water at a final concentration of 1 g L 1 . Additional experi- ments were performed with MgSO 4 (Merck, Germany) and NaH 2 PO 4  2H 2 O (RDH Laborchemikalen, Germany) as substitutes of MgCl 2  6H 2 O and Na 2 HPO 4  2H 2 O, respectively. Turbidity of TiO 2 suspensions before and after struvite compo- nents addition were measured with turbidemeter Model 2100P (HACH, Germany) (calibration curve is shown in Fig. 1). 250 ml flasks were filled with 150 ml TiO 2 P-25 suspension in distilled water (1 g L 1 ) at room temperature (22 ± 2 °C). Under continuous magnetic stirring, 10, 33 and 40 ml of each struvite components were added in different orders (see Table 1) and left to flocculate 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.03.015 * Corresponding author. E-mail address: cvrrobi@tx.technion.ac.il (R. Armon). 1 Member of Grant Water Research Institute, Technion, Haifa, Israel. Journal of Colloid and Interface Science 336 (2009) 107–110 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis