Adsorption of Molybdate Anion (moo4 2-) by Sodium-saturated Kaolinite

-Adsorption of Mo(VI) on 2-0.2-um size fraction of sodium-saturated kaolinite at 25 _+ 2~ and at a constant pH of 7.00 _+ 0.05 was studied. The kaolinite sample was pretreated to remove any surface oxide and hydroxide coatings. The initial concentrations of Mo in solution ranged from 1 to 11 mg/liter in a NaC104 background electrolyte at a constant ionic strength of 0.09 _+ 0.01. Calculations of speciation using the GEOCHEM computer program indicated that under experimental conditions Mo(VI) was mainly in the MoO4 2form. The experimental conditions were also shown to fulfill the requirements for applying the Langmuir equation in interpreting adsorption data. The Langmuir parameter for the adsorption maximum, n ~ and the affinity parameter, KMoO,2 -ao, were computed to be 3.33 x 10 -4 mole/ mole of adsorbent and 5.969 x 105, respectively. The large affinity parameter indicated that the Nasaturated kaolinite surface has a very high affinity for MoO42 ions relative to C10 4ions. Key Words--Adsorption, Electrolyte, Kaolinite, Molybdate ion, Speciation. I N T R O D U C T I O N Molybdenum is an essential trace element for plant and animal life; however, excessive amounts can have deleterious effects on various organisms. According to Aubert and Pinta (1977), total Mo concentrations in soils as high as 24 ppm have been found. A survey by Kopp and Kroner (t970) indicated that the surface waters in the United States contain Mo concentrations between 0.06 and 1.10 mg/liter. Concentrations of Mo in the environment are significantly enhanced by anthropogenic inputs from coal-resource development, fly ash, sewage sludge, and hard-rock mining activity. Sediments downstream from a molybdenum mine were reported to contain Mo concentrations as high as 2000 ppm (Vlek and Lindsay, 1977); the dissolved Mo in stream water was as high as 3.8 mg/liter (Jorden and Meglen, 1973). Therefore, it is essential to understand various Mo interactions in soils and sediments in order to assess the effects ofanthropogenic inputs of this trace element into natural waters and soil systems. An important aspect of Mo interaction in soils and sediments is its adsorption on to clay mineral surfaces. Relatively few studies of Mo adsorption on phyllosilicates have appeared in the literature, and only two studies have been published that involve the adsorption of Mo on kaolinite (Barshad, t 951; Jones, 1957). These studies were conducted using kaolinite samples that were not characterized, and background electrolytes were not used to maintain constant ionic strengths. In addition, these studies used a single initial concentration of Mo with variable pH. Therefore, a singular lack of data exists regarding Mo adsorption on wellcharacterized kaolinite with varying initial concentrations of Mo in constant ionic strength solutions. The objective of the present study was to obtain such data Copyright 9 1984, The Clay Minerals Society 45 under clearly defined conditions of Mo speciation in equilibrium solutions at constant pH. MATERIALS A N D METHODS Preparation of kaolinite suspension A sample of well-crystallized kaolinite from Georgia designated as KGa-1, obtained from the Source Clays Repository of The Clay Minerals Society, was dispersed in 0.1 M NaOH solution, and the 0.2-2-tzm size fraction was separated by centrifugation. This size fraction was reacted with 0,001 M Hf lO4 in 1.0 M NaC104 solution, washed several times with 0.1 M NaC104 solution at neutral pH, and stored as a suspension in the same electrolyte in a polyethylene container. This pretreatment procedure was used to remove any oxide and hydroxide coating on kaolinite surfaces. The Na-saturated kaolinite was used in all of the adsorption experiments. The weight of clay per unit volume of suspension was determined by drying known volumes of suspensions at 110~ for 24 hr and applying appropriate corrections for occluded salt. Quadruplicate measurements indicated that the suspension contained 373.14 • 1.80 mg/ml of clay. A sample of suspension washed free of electrolyte was dissolved in aqua regia and hydrofluoric acid following the method of Bernas (1968). The analysis of the resulting solution indicated that the kaolinite sample contained no detectable molybdenum. Adsorption experiments To study the adsorption o f Mo by Na-kaolinite, solutions of Na2MoO4 ranging from 1 to 11 rag/liter were added to polyethylene centrifuge tubes containing 15 ml of kaolinite suspensions. Aliquots of 0.1 M NaC104 solution were added to each centrifuge tube to maintain 46 Phelan and Mattigod Clays and Clay Minerals Table 1. Association constants for aqueous molybdenum species at 25~ and 1 atmosphere.