Efficiency And Mechanism Of Molybdenum Removal From Contaminated Water By In Situ Iron(Hydro) Oxides

Molybdenum is a necessary trace element for both plants and animals and it is also a kind of important resource. Nowadays molybdenum is widely used in steel industry, oil and chemical industry, composite materials field, agriculture field and medical field. As the exploration of molybdenum mines, the molybdenum pollution problem is becoming more and more serious. Subchronic and chronic oral exposure to molybdenum can result in gastrointestinal disturbance, growth retardation, anemia, hypothyroidism, bone and joint deformities, sterility, liver and kidney abnormalities and death. So it is necessary to develop proper technology to remove molybdenum from water. Coagulation-sedimentation-filtration is a traditional process being used in the water treatment engin eering, and it is characterized by low cost, high efficiency and easy to operate. Researches show that it is efficient to remove many kinds of metal ions from water. So this paper used different kinds of iron salts as coagulants and investigate the effect of the in situ iron(hydro) oxides derived from the hydrolysis of the iron salts on the removal of molybdenum, study the influence of various affecting factors and search for the removal mechanisms.Results show that the in situ iron(hydro) oxides derived from the hydrolysis of Fe Cl3、Fe2(SO4)3 and X-Fe2(SO4)3 all showed a high efficiency on the removal of Mo(VI). Under pure water background condition, the Mo(VI) removal efficiency of the three iron salts all increased with the increase of p H firstly, and then decreased with the increase of p H. The effect of iron salts on the removal of Mo(VI) at different p Hs was dominated by three factors:(1) The content of Fe intercepted from water;(2) The Zeta potential of the iron flocs;(3) The species of the Mo(VI).During the factors which may influence of the removal of Mo(VI), solution temperature and ion strength had little influence on the removal of Mo(VI) by the iron salts; Ca2+ had little influence on the removal of Mo(VI) by Fe Cl3 and X-Fe2(SO4)3, but could decrease the removal of Mo(VI) by Fe2(SO4)3. The influence of co-existing anions(including humic acid) on the removal of Mo(VI) by the iron salts was dominated by three factors:(1) the influence of co-existing anions on the content of Fe intercepted from water;(2) the competition of co-existing anions with Mo(VI) for adsorption sites;(3) the influence of co-existing anions on the Zeta potential of the iron flocs.The mechanisms of Mo(VI) removal by Fe2(SO4)3 were investigated. It was found that the Mo(VI) was adsorbed onto the surface of the Fe2(SO4)3 flocs and formed inner-sphere complex. At low Fe2(SO4)3 dosage, no coprecipitation effect happened, while at high Fe2(SO4)3 dosage, the coprecipitation effect contributed to the removal of Mo(VI).Results show that X1.0-Fe SO4 solution(also named X-Fe2(SO4)3 solution) had a much higher Mo(VI) removal efficiency compared with pure Fe SO4 solution, its Mo(VI) removal efficiency was similar to Fe2(SO4)3 solution and its Mo(VI) removal efficiency didn’t decrease as the increase of aging time. The reasons for this was that 90% of the Fe2+ was oxidized to Fe3+ in the X1.0-Fe SO4 solution, and the acidity of the X1.0-Fe SO4 solution was very high with the addition of reagent X which stopped the hydrolysis of the Fe3+.Under natural water background condition, the Mo(VI) removal efficiency of the three iron salts were approximately the same, and they all showed a decrease tendency as the increase of p H. To guarantee the high Mo(VI) removal efficiency and reduce the dosage of the p H adjustment reagent, it is better to adjust the coagulation p H to 5.0 in the practical water treatment engineering, and adjust the p H back to neutral after the Mo(VI) was removed. When the initial Mo(VI) concentration was 0.7 mg/L, 15 mg/L Fe was needed for the three iron salts to make the Mo(VI) concentration of the treated water low enough to meet the requested value of the “Standards for drinking water quality(GB5749-2006)”. When adding 15 mg/L Fe, the reagent cost of the Fe Cl3 、 Fe2(SO4)3 and X-Fe2(SO4)3 process was 0.2229 yuan per ton water, 0.3403 yuan per ton water and 0.059 yuan per ton water, separately.