liters of water to obtain a solution containing 500 mg l of lead iron iii chloride was added to the solution with stirring for 5 min to obtain a mixed solution containing 400 mg l of iron and the ph of the mixed solution was adjusted to 9 with a naoh solution the mixed solution was stirred for 30 min to allow precipitation of an iron hydroxide together with a lead hydroxide next a polymer coagulant a polyacrylamide base coagulant was added in the proportion of 1 mg l and the mixture was further stirred for 5 min after that the mixture was left standing for 4 h to allow sedimentation of solid matter then a supernatant liquid of the mixture was removed to leave a slurry sediment containing lead using this sediment the following tests were carried out the test procedure was generally similar to the elution test procedure in example 1 except that the filtrate of the eluate was subjected to the measurement of the concentration of lead by the atomic absorption spectrometry according to jis k 0102 the results are shown in fig5 it is seen that both acidic conditions below about 6 in ph value and basic conditions above about 11 in ph value are favorable for the elution of lead from the iron hydroxide sediment the test procedure was fundamentally analogous to the test procedure in example 1 a sulfuric acid solution and water were added to 100.