Earth and Planetary Science Letters, 22 (1974) 366-370 0 North-Holland Publishing Company, Amsterdam - Prmted m The Netherlands zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPON FRACTIONATION OF SULFUR ISOTOPES BY SEDIMENT ADSORPTION OF SULFATE JEROME 0. NRIAGU Department of the Enwonment, Canada Centre for Inland Waters, Burhngton, Ont. (Canada) Received October 4, 1973 Revised version received April 5, 1974 Adsorptlon studies with sediment from the Bay of Qumte (Lake Ontario) mdlcate that the adsorption isotherm for sulfate concentration greater than 20 mg/l can be described by a Langmmr-type equation The accompanymg changes m 634S of the solutions range from +0.9 to +6.0%0 indicating enrichment of “‘S m the sediments The cor- responclmg isotopic fractionation between the adsorbed sulfate and the sulfate remaining m solution ranges from - 10 to -24%c, the larger fractionations being associated with low sulfate concentrations The adsorbed sulfate IS shown to significantly afftect the isotopic composition of the sediment m contact with the sulfate-bearmg solution The de- sorption of adsorbed sulfate is considered a possible mechanism for enriching natural freshwaters wtth “S. 1. Introduction Geochemical studies relating to the distribution and dynamics of sulfur in aquatic ecosystems show that m most instances, the sediments act as the sink for the sulfur (e.g., see [ 1] ). The mechanism commonly invok- ed to explain the enrichment of sulfur m sediments mvolves the blologlcally-mediated reduction of sulfate (the dominant form of sulfur in natural waters) follow- ed by the immobilization of the sulfur as elemental sulfur or metal (mostly iron) sulfides. The fractlona- tlon of sulfur isotopes between the various sulfur-bear- ing phases m the ecosystem 1s widely believed to be engendered entirely by the microbial activities. Nume- rous studies, however, have recorded adsorptive reten- tion of significant concentrations of sulfate by various sol1 types and soil constituents [2-61. Little, if any, information is available on the adsorption and desorp- tlon of sulfate by sediments or on the isotopic effects of the adsorption process. The objectives of this study have been (a) to exa- mme the role of adsorption processes on the retention of sulfate by freshwater sediments, and (b) to assess the effects of sulfate adsorption on the isotopic com- posltlon of the sulfate m the aqueous phase. 2. Experiment The sediment used m the study was an organic-rich (11.5% organic carbon), low-carbonate clayey mud from the Bay of Quinte, Lake Ontario. Before each run, the dried sediment was washed several times to re- motie any sulfate formed by the oxidation of amor- phous iron sulfides. For adsorption studies, 10 g of the washed sediment was added to 100 ml of the so- dium sulfate solution and the mixture shaken continu- ously for 48 hours at room temperature (23 5 2°C). At the end of each run the mixture was filtered through a 0.45-p Mlllipore membrane and the sulfate in aliquots of the filtrate determined gravimetrically as BaS04. Total sulfur in the starting material was determined by fusion with Eschka mixture (2 parts MgO and 1 part anhydrous Na2C03) followed by precipitation of the sulfate in the leachate as BaS04. The barium sulfate was converted to SO* by the thermal decomposition procedure reported by Holt and Engelkemeir [7]. The 34S/32S ratio m the SO2 was determined on a dual-collector isotope-ratio mass spectrometer (VG Micromass, Model 602C). The “s/32S ratios of SO* samples from adsorption studies were compared against the isotopic ratio of SO2 from