So I read this paper, and found something valuable there. Photocatalysis is often used for treating water organic pollutants. Titanium dioxide is usually used because it is stable and inexpensive. This paper stated that one author observed that increasing the initial pH of phenol, increases the decomposition rate, reaching max value at 3.3, then decreasing upon further increase in pH.
Another observer observed for 3-chlorophenol. Basically, they indicated almost the same idea. The degradation rate will decrease if the soution pH is increased. In another term, the removal efficiency will become higher if the solution pH is increased, because higher removal efficieny means lower (faster) degradation rate. The idea of this paper was to study more the effect of pH for chlorophenol degradation with TiO2-UV.
From this paper, I know that IE (Isoelectric) point of TiO2 is 6.3. But the HA he used was purchased from Fluka, which different from HA I purchased from Aldrich. I note the result, under dark condition, there was no adsorbtion occured for chlorophenols. Although, there was little bit adsorption on very low pH for several minutes after the starting, desorption phenomena began. It was also reported that under UV illumination, higher pH, TCP (trichlorophenols) degradation was more rapid.
Relationship between TiO2 performance with pH
Senogles stated in his paper that TiO2 performance is affected by pH. High pH tends to form positive charge in titanium dioxide surface while low pH will negatively charge it. I don’t know exact pH which will make positive or negative charge. However, this “charge things” definitely affect the performance. Why, because it depends on the solution charge. Suppose the feed water is dominantly negative, while the pH is high, positive charge for TiO2 more adsorption occurs because of electrostatic attraction. However, this is still a possibility. The fact is he did a research under dark condition, try to find out whether adsorption occurs or not for cylindrospermopsin as a function of pH. The result is, no significant adsorption, no UV no adsorption, pH change does not give adsorption.
In my research, I found that humic acid particles are adsorbed by TiO2 particles proven by photocatalysis batch. Although it was only 8%, but still there was removal via adsorption. The only difference with above is that, the photocatalysis batch was illuminated by UV254 and my parameter is humic acid in terms of dissolved organic carbon (DOC). So at this point, I think there is a point when actually UV254 helps organic matter removal for adsorption. I did not do photocatalysis batch without illumination. I did only 5 or 10 minutes, but I think that is not significant to prove that UV254 helps organic adsorption in the surface of titanium dioxide.
Again, back to Senogles et al (2001), he discovered that under illumination of UV, greatest degradation of cylindrospermopsin was observed for high pH (9), approx 90-95%, while on pH 4 around 75-80%. He had mentioned that, without illumination (irradiation) of UV, no adsorption was found, thus with irradiation he concluded that the degradation was mainly due to mechanism involving dissolved organic radicals. To read this conclusion, I have one question arise. Indeed that under dark condition, no adsorption was found as a function of pH. But that doesnt mean that under illumination or irradiation of UV, no adsorption occur. Indeed, the change of pH, from 4 to 9, degradation rate 70-75% to 90-95%, I believe 20% absolutely from dissolved organic radicals (conversion), but that doesn’t mean the remaining was totally from conversion. Because he did not do further experiment whether under irradiation there was adsorption or not in TiO2 particles. Why I say this, because I found for humic acid solution under UV irradiation, adsorption occured taking into account approx 8% while 15% due to conversion.
Tanaka, S. and U. K. Saha. Effects of pH on photocatalysis of 2,4,6-trichlorophenol in aqueous TiO2 suspensions. Wat. Sci. Tech. 30 (9) (1994) 47-47.
Senogles, Wat Res (2001)