Chromate adsorption on amorphous iron oxyhydroxide was investigated in dilute iron suspensions as a single solute and in solutions of increasing complexity containing C02(g), SOZ-(aq), H4Si04(aq), and cations [K+, Mg2+, Ca"(aq)]. In paired-solute systems (e.g., Cr0:-H2C03*), anionic cosolutes markedly reduce Cr042-adsorption through a combination of competitive and electrostatic effects, but cations exert no appreciable influence. Additionally, H4Si04 exhibits a strong time-dependent effect: Cr0:-adsorption is greatly decreased with increasing H4Si04 contact time. In multiple-ion mixtures, each anion added to the mixture decreases Cr0:-adsorption further. Adsorption constants for the individual reactive solutes were used in the triple-layer model. The model calculations are in good agreement with the Cr042-adsorption data for paired-and multiple-solute systems. However, the model calculations underestimate Cr042-adsorption when surface site saturation is appr6ached. Questions remain regarding the surface interactions of both C02(aq) and H4Si04. The results have major implications for the adsorption behavior of Cr0:-and other oxyanions in subsurface waters.

The adsorption of quinoline and aminonaphthalene was investigated from aqueous solution on amorphous silica @io2). Amorphous Si02 was not a strong adsorbent of these compounds and quinoline was adsorbed more strongly than aminonaphthalene. The adsorption of both compounds varied with pH. A maximum in adsorption occurred near their respective pK,'s. Temperature effects were significant, suggesting enthalpy contributions in the range of H bonding. Both compounds appeared to adsorb via identical mechanisms that included H bonding and ion exchange. The greater basicity of quinoline combined with (1) the enhanced electron-donating properties of the heteroatom N and (2) the delocalized .Ir-bonded ring system were proposed to account for its stronger adsorption. Aminonaphthalene showed greater exclusion from the internal region of porous Si02, indicating that the compounds orient differently at the surface. The triple-layer adsorption model and three outer-sphere surface complexes were used to model the adsorption data. Good predictions of adsorption isotherms and the effects of ionic strength and electrolyte cation were obtained. Experimental ProceduresSorbates. 1-Aminonaphthalene and quinoline (99% purity, Aldrich Chemical Co.) were used without further purification for sorbate stock solutions. These stock solutions were spiked with 14C-labeled aminonaphthalene or quinoline (Sigma Chemical Co.) that had been purified by high-pressure liquid chromatography (HPLC). The radiochemical purity of the 14C-labeled compounds was >99% and their specific activity was approximately 10 mCi mmol-'. Selected physicochemical properties of the sorbate molecules are summarized in Table I.Porous and Amorphous SiOp Microporous, amorphous, Merck silica gel [Si02(ap), Aldrich] was prepared by first washing in acid (0.1 M HC1) and then repeatedly (-10 times) in distilled/deionized water until chloride free. The SiO,(ap) was then dried at 100 "C. The microporous silica gel was reported by the supplier to have a surface area of 675 m2 g-', a particle size of 212-500 pm, a pore size of 4.0 nm, and a pore volume of 0.68 cm3 g-'.Nonmicroporous, amorphous Si02 [Si02(ac)] was prepared by adding dry Aerosil200 (Degussa, Inc.) to 0.01 M NaCl to yield a suspension with a 1:lO solids to solution ratio. The Si02(ac) had a surface area of 183 m2 g-', a particle size of 0.012 pm, and no internal pore space.General Experimental Procedures. Sorption experiments were conducted in an incubator-shaker controlled to within 0.5 "C of the desired temperature, which was usually 25 "C. Preweighed 25-mL Corex centrifuge tubes, sealed with Teflon-lined silicon rubber septa, were used for all equilibrations. Sorption experiments were performed with a single sorbate concentration over a range in pH (sorption edges) and with variable sorbate concentration at a single pH (isotherms). Unless otherwise indicated, the electrolyte was 0.01 M NaCl and the titrants for controlling pH were 0.1 M NaOH and 0.1 M HC1. The sorbent concentration was 50 g of Si02 L-' (1:20 solids to...

Studies of the relationship between limnological factors and changes in the P status of noncalcareous sediments of a eutrophic lake previously indicated that changes in sediment inorganic P were directly related to the biological productivity of the surface waters. This relationship was particularly pronounced in a shallow bay where sediment P decreased with lake phytoplankton growth and increased with their decline. The present studies were undertaken to identify the inorganic P components responsible for these changes using chemical techniques for characterization of P in sediments where changes occurred.Release of P from sediments during periods of maximal biological productivity in the shallow bay waters was due primarily to reductions in inorganic P mineral components soluble in NaOH (0.1M) and citrate‐dithionite, suggesting that P associated with hydrated Fe oxides was the principal source of P to the overlying waters and to the biota. Increases in sediment inorganic P, which occurred when phytoplankton growth diminished, were due principally to increases in P mineral components soluble in NH4F, indicating that resorbed P was F‐exchangeable and/or associated with crystalline and amorphous Al‐P components. Changes in the form of sediment P which occurred in the lake and in laboratory equilibrium studies, indicated that conversion of resorbed NH4F soluble‐P to NaOH (0.1M) and citrate‐dithionite soluble‐P, must be a relatively long term (>2 years) process. Taken together, the NH4F, NaOH (0.1M), and citrate‐dithionite fractions accounted for approximately 85% of the seasonal variance in total inorganic P which occurred at all sampling locations in the lake system.

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