(a) silicon dioxide, (b) at least one basic substance selected from the group consisting of an inorganic salt of an alkali metal, an ammonium salt, piperazine and ethylenediamine, (c) at least one chelating agent selected from the group consisting of a compound represented by the following general formula [1] and its salt: wherein each of R1 and R2 which are the same or different, is a lower alkyl