Synthesis of PPy-modified TiO2 composite in H2SO4 solution and its novel adsorption characteristics for organic dyes

• The novel PPy/TiO2 composite has a potential to treat dye wastewater.
• The H2SO4 concentration affects the adsorption ability of the composites.
• The selective adsorption by PPy/TiO2 can be complete by changing its surface charge.
• The equilibrium adsorption is achieved in a short time of 30 min.
• The composite adsorbent can be reused repeatedly with a high adsorption capacity.

The PPy/TiO2 and PPy/P25 composites prepared via the in situ polymerization in H2SO4 solution with different concentrations were studied for Acid Red G (ARG) or Methylene Blue (MB) adsorption. Their physicochemical properties were examined by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA) and zeta potential analysis. The results showed that the H2SO4 concentration and the property of metal oxide (self-prepared TiO2 or commercial P25) obviously affected the nature and adsorption abilities of the PPy/TiO2 and PPy/P25 composites. The composites can adsorb Acid Red G (ARG) or Methylene Blue (MB) rapidly and reach the equilibrium in 30 min. The adsorption amount of ARG on the composites decreased with the increase of pH value, while the MB removal efficiency increased with pH increase. The adsorption behaviors of ARG and MB on the composites were described by the pseudo-second-order and Langmuir isotherm models. According to the Langmuir isotherm, the PPy/TiO2 composites exhibited larger maximum adsorption amount than PPy/P25, especially with the PPy/TiO2 composite prepared in 0.16 mol/L H2SO4 (0.16 M-PPy/TiO2), on which the maximum adsorption amounts of ARG and MB were 218.82 and 323.62 mg/g, respectively. Regeneration experiments revealed that the 0.16 M-PPy/TiO2 composite can be regenerated easily and reused for six times without visible loss of its original capacity. Additionally, its adsorption efficiencies of ARG and MB in real effluents were still higher than 87% after adsorption–desorption for four times. Through comparing the FT-IR spectra of PPy/TiO2 before and after adsorption, as well as the surface charge measurement, it can be speculated that electrostatic interaction, hydrogen bonding and specific chemical interaction may be the possible mechanism for the adsorption of ARG and MB by the PPy/TiO2 composites.

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