2.2. Supernatant measurements
PH and temperature of the samples were measured at the end of the flocculation process, using a pH 3310 portable pH meter (WTW). With a 2100p turbidimeter (HACH), turbidities of the supernatant in the flocculation jars were measured and recorded as residual turbidities (RT). Titanium ion concentrations in the supernatant were usually determined by the salicyl-fluorone spectrophotometric method (Ministry of Health of the People’s Republic of China, 2007) with a UV spectrophotometer (Model UV765, Shanghai Precision & Scientific Instrument Co.), and concentrations lower than 0.02 mg Ti/L were measured by inductively coupled plasma mass spectrometry (ICP-MS), using an ELAN DRC-e spectrometer (Perkin–Elmer).
2.3. Floc property measurements
SEM images of flocs were provided by a Quanta 200F (FEI) electron microscope, and TEM images were obtained with a Tecnai G2 F20 (FEI) electron microscope. Floc size distributions were determined using an Eyetech particle analyzer (Ankersmid). Floc zeta potentials were measured with a Zetasizer Nano Z analyzer (Malvern). Representative settling velocities of flocs were measured in a transparent cylinder by the video recording method. Floc densities were estimated by observing the buoyancy of floc particles released in a series of salt solutions with different densities.
Settled flocs were collected and vacuum-filtered onto 0.45 μm pore-size poly-vinylidene fluoride (PVDF) membranes. The filtered flocs were stored under room temperature in a desiccator for one week before further analyses. The TG and DSC analyses were conducted on a Q600 SDT analyzer (TA) with an air flow of 100 mL/min and a temperature ramp of 5 °C/min. Floc composition was analyzed by XRF method using an XRF-1800 spectrometer (Shimadzu). XRD analyses were performed on a D8 Advance (Bruker-AXS) diffractometer.