Removal of Molybdate Anions from Water by Adsorption on Zeolite-Supported Magnetite Bram Verbinnen 1* , Chantal Block 1 , Dries Hannes 1 , Patrick Lievens 1,2 , Miroslava Vaclavikova 3 , Katarina Stefusova 3 , Georgios Gallios 4 , Carlo Vandecasteele 1 ABSTRACT: Industrial wastewater may contain high molybdenum concentrations, making treatment before discharge necessary. In this paper, the removal of molybdate anions from water is presented, using clinoptilolite zeolite coated with magnetite nanoparticles. In batch experiments the influence of pH, ionic strength, possible interfering (oxy)anions, temperature and contact time is investigated. Besides determination of kinetic parameters and adsorption isotherms, thermodynamic modeling is performed to get better insight into the adsorption mechanism; molybdenum is assumed to be adsorbed as a FeOMoO 2 (OH).2H 2 O inner-sphere complex. At the optimum pH of 3, the adsorption capacity is around 18 mg molybdenum per gram adsorbent. The ionic strength of the solution has no influence on the adsorption capacity. Other anions, added to the molybdenum solution in at least a tenfold excess, only have a minor influence on the adsorption of molybdenum, with the exception of phosphate. Adsorption increases when temperature is increased. It is demonstrat- ed that the adsorbent can be used to remove molybdenum from industrial wastewater streams, and that the limitations set by the World Health Organization (residual concentration of 70 lg/l Mo) can easily be met. Water Environ. Res., 84, 753 (2012). KEYWORDS: molybdenum; zeolite-supported magnetite nanoparticles, adsorption, wastewater treatment, molybdate, oxyanions. doi:10.2175/106143012X13373550427318 Introduction Molybdenum and also arsenic, chromium, antimony, seleni- um, vanadium and tungsten usually occur in solution as oxyanions, containing a central metal or metalloid with several oxygen atoms around it and carrying a negative net charge. So far, (environmental) interest was mainly focused on arsenic, the number one priority pollutant according to U.S. EPA, and on chromium, but nowadays other oxyanions are also considered as ‘‘emerging pollutants’’ (Vandecasteele and Cornelis, 2010). Molybdenum is mainly used in metallurgical applications as an alloying element in the production of stainless steel or cast- iron alloys (Barceloux 1999), and in the production of flame retardants, pigments and catalysts for high temperature chemical processes. The molybdate ion MoO 4 2- , the most common oxyanion of molybdenum, occurs in various types of wastewaters, like wastewater from a styrene monomer plant (1000 mg/l) (Swinkels et al., 2004), scrubber effluent of a municipal solid waste incinerator (0.95 mg/l) (Lievens et al., 2010) and mining water (, 0.1 to 2.2 mg/l) (Dessouki et al., 2005; Panayotova and Panayotov, 2004; Rojas and Vandecasteele, 2007). Traditional industrial wastewater treatment plants remove cations (e.g. Cu 2 þ , Zn 2 þ ) by precipitation as their hydroxides. However, this process does not remove oxyanions, so that they may still be present in the effluent (Lievens et al. 2010). Near industrial sources, such as molybdenum mining areas, the molybdenum concentration in surface water may reach concentrations of 200 to 400 lg/l and in groundwater 25 mg/l (Barceloux, 1999). These high concentrations pose health problems to people living in the neighbourhood of mining areas and using well water. Subchronic and chronic oral exposure can result in gastrointestinal disturbances, growth retardation, anaemia, hypothyroidism, bone and joint deformities, sterility, liver and kidney abnormalities, and death (Namasivayam and Sangeetha, 2006). The guideline for molybdenum in drinking water established by the World Health Organization is 70 lg/l. In January 2011, a limit value of 340 lg/l molybdenum was included in the ‘‘basic quality standards for surface water’’ in Flanders (the northern part of Belgium). Molybdenum concen- trations in effluent discharges may not exceed this value. Therefore, in order to comply with such legislation, techniques to remove molybdenum from mining water, industrial effluents, ground and surface water are needed. Most studies on removal of oxyanions from water focus on arsenic in drinking water or in synthetic solutions. However, a number of the methods described for the removal of oxyanions, cannot be applied or are difficult to apply for most process waters and wastewaters, because the residual concentrations are too high (precipitation), because toxic compounds remain after treatment (biological methods), or because clogging and fouling occur unless elaborate pretreatment is applied (ion exchange, membrane techniques). At high concentrations, oxyanions can be precipitated as their Ca-metalates, but this usually leaves rather high residual concentrations (Swinkels et al., 2004). Adsorption on goethite 1 University of Leuven, Laboratory of Applied Physical Chemistry and Environmental Technology, Department of Chemical Engineering, K.U. Leuven, W. De Croylaan 46, B-3001 Leuven, Belgium. 2 Leuven Engineering College Groep T, Department of Chemical Engineering, Vesaliusstraat 13, 3000 Leuven, Belgium. 3 Institute of Geotechnics, Slovak Academy of Sciences, Watsonova 45, SK-043, Kosice, Slovakia. 4 Laboratory of General and Inorganic Chemical Technology, School of Chemistry, Aristotle University of Thessaloniki, Gr-540 06, Thessaloniki, Greece. * University of Leuven, Laboratory of Applied Physical Chemistry and Environmental Technology, Department of Chemical Engineering, K.U. Leuven, W. De Croylaan 46, B-3001 Leuven, Belgium; e-mail: bram.verbinnen@cit.kuleuven.be. September 2012 753