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(supplied courtesy of Cabot Corp., Boston, MA), which consists predominantly of 7-A1203 (16). Alon has previously been used as a model oxide surface in studies of adsorption of organic matter and some trace metals onto particle surfaces (16,17). Alon particles are reported to be nonporous and approximately spherical with a specific surface area of 120 m2 g-1 (17). Scanning electron microscopy showed that the Alon particles provided for this study consisted of particles with a range of diameters from 0.04 - 0.5 pm. Consequently, to ensure addition of only > 0.2 pm particles in the experiments, a cleaned Alon suspension (17) was fractionated by a series of filtrations. Alon was added to experiments to give particle concentrations typical of near-shore surface waters (0.1 - 5.0 mg 1-1). Particle concentration was monitored by light scattering measurements using a fluorometer. After 25 days, no significant change in scattering was measured in the photo-oxidized seawater samples. A decrease in scattering of < 16% was observed with time in the 1.0 mg C1-1 samples however. This decrease may be due to dissolution of the alumina particles or to the attachment of particles to the walls of the bottles. RESULTS To determine the effect of colloidal matter on the equilibrium partitioning of Th, 234Th spikes were first added to Alon suspensions that had been prepared in colloid-free seawater (ultrafiltered through a 1, 000 NMW filter). Figure 1 shows the partitioning results for Alon suspensions (0 - 5.0 mg 1-i) in colloid-free seawater with dissolved organic matter (DOM) concentrations of 0.1 mg C1-1 (Figure la) and 1 mg C1-1 (Figure ib). Separation of the samples into dissolved (< 1, 000 NMW), colloidal (1, 000 NMW-0.2 pm), and particulate (> 0.2 pm) fractions showed that with increasing Alon concentration, colloidal Th (The) decreased concurrently with an increase in particulate Th (Thy ). Alon concentration had little effect on the percentage of Th in the dissolved fraction (Thd). The colloidal fraction of 234 Th was found, by sequential filtrations, to pass ultra-filters with nominal molecular weight cut-offs of > 5, 000 and to be retained by the YM2 ultrafilter (1, 000 NMW). Under the conditions of Figure 1 (i. e. colloid-free seawa-ter), hydrolysis complexes and organic complexes are the only species of Th present in solution (13). Therefore, the colloidal 234 Th measured in the photo-oxidized seawater samples (Figure la) was assumed to consist of Th hydroxide complexes. The colloidal Th in the 1.0 mg C1-1 DOM samples (Figure lb) was assumed to be predominantly bound in hydroxides as well since < 5% of the total sample 14C was detected in the colloidal fraction. This concentration of Phaeodactylum tricornutum exudates (< 0.05 mg C1-1 ) would complex < 3% of the total Th in the sample (7). Amicon YM2 ul-trafilters have been reported to retain species with molecular weights much lower than their nominal molecular weight cut-off of 1, 000 (down to 200 MW) and to have a 50% retention at 380 MW (18). Therefore, these hydroxide complexes may be Th(OH)4, the dominant species of the Th in seawater (13), or polynuclear hydroxide complexes. The dissolved Th species in Figure 1 are probably organic complexes or hydrolysis complexes not retained by the YM2 ultrafilter. Phaeodactylum tricornutum exudates have been found to form strong complexes with Th; at 1.0 mg C 1 -1, 47% of total Th is bound in dissolved organic complexes (7). From a study of Th adsorption by metal oxides in seawater, Hunter et al. (19)
DOI link for (supplied courtesy of Cabot Corp., Boston, MA), which consists predominantly of 7-A1203 (16). Alon has previously been used as a model oxide surface in studies of adsorption of organic matter and some trace metals onto particle surfaces (16,17). Alon particles are reported to be nonporous and approximately spherical with a specific surface area of 120 m2 g-1 (17). Scanning electron microscopy showed that the Alon particles provided for this study consisted of particles with a range of diameters from 0.04 - 0.5 pm. Consequently, to ensure addition of only > 0.2 pm particles in the experiments, a cleaned Alon suspension (17) was fractionated by a series of filtrations. Alon was added to experiments to give particle concentrations typical of near-shore surface waters (0.1 - 5.0 mg 1-1). Particle concentration was monitored by light scattering measurements using a fluorometer. After 25 days, no significant change in scattering was measured in the photo-oxidized seawater samples. A decrease in scattering of < 16% was observed with time in the 1.0 mg C1-1 samples however. This decrease may be due to dissolution of the alumina particles or to the attachment of particles to the walls of the bottles. RESULTS To determine the effect of colloidal matter on the equilibrium partitioning of Th, 234Th spikes were first added to Alon suspensions that had been prepared in colloid-free seawater (ultrafiltered through a 1, 000 NMW filter). Figure 1 shows the partitioning results for Alon suspensions (0 - 5.0 mg 1-i) in colloid-free seawater with dissolved organic matter (DOM) concentrations of 0.1 mg C1-1 (Figure la) and 1 mg C1-1 (Figure ib). Separation of the samples into dissolved (< 1, 000 NMW), colloidal (1, 000 NMW-0.2 pm), and particulate (> 0.2 pm) fractions showed that with increasing Alon concentration, colloidal Th (The) decreased concurrently with an increase in particulate Th (Thy ). Alon concentration had little effect on the percentage of Th in the dissolved fraction (Thd). The colloidal fraction of 234 Th was found, by sequential filtrations, to pass ultra-filters with nominal molecular weight cut-offs of > 5, 000 and to be retained by the YM2 ultrafilter (1, 000 NMW). Under the conditions of Figure 1 (i. e. colloid-free seawa-ter), hydrolysis complexes and organic complexes are the only species of Th present in solution (13). Therefore, the colloidal 234 Th measured in the photo-oxidized seawater samples (Figure la) was assumed to consist of Th hydroxide complexes. The colloidal Th in the 1.0 mg C1-1 DOM samples (Figure lb) was assumed to be predominantly bound in hydroxides as well since < 5% of the total sample 14C was detected in the colloidal fraction. This concentration of Phaeodactylum tricornutum exudates (< 0.05 mg C1-1 ) would complex < 3% of the total Th in the sample (7). Amicon YM2 ul-trafilters have been reported to retain species with molecular weights much lower than their nominal molecular weight cut-off of 1, 000 (down to 200 MW) and to have a 50% retention at 380 MW (18). Therefore, these hydroxide complexes may be Th(OH)4, the dominant species of the Th in seawater (13), or polynuclear hydroxide complexes. The dissolved Th species in Figure 1 are probably organic complexes or hydrolysis complexes not retained by the YM2 ultrafilter. Phaeodactylum tricornutum exudates have been found to form strong complexes with Th; at 1.0 mg C 1 -1, 47% of total Th is bound in dissolved organic complexes (7). From a study of Th adsorption by metal oxides in seawater, Hunter et al. (19)
(supplied courtesy of Cabot Corp., Boston, MA), which consists predominantly of 7-A1203 (16). Alon has previously been used as a model oxide surface in studies of adsorption of organic matter and some trace metals onto particle surfaces (16,17). Alon particles are reported to be nonporous and approximately spherical with a specific surface area of 120 m2 g-1 (17). Scanning electron microscopy showed that the Alon particles provided for this study consisted of particles with a range of diameters from 0.04 - 0.5 pm. Consequently, to ensure addition of only > 0.2 pm particles in the experiments, a cleaned Alon suspension (17) was fractionated by a series of filtrations. Alon was added to experiments to give particle concentrations typical of near-shore surface waters (0.1 - 5.0 mg 1-1). Particle concentration was monitored by light scattering measurements using a fluorometer. After 25 days, no significant change in scattering was measured in the photo-oxidized seawater samples. A decrease in scattering of < 16% was observed with time in the 1.0 mg C1-1 samples however. This decrease may be due to dissolution of the alumina particles or to the attachment of particles to the walls of the bottles. RESULTS To determine the effect of colloidal matter on the equilibrium partitioning of Th, 234Th spikes were first added to Alon suspensions that had been prepared in colloid-free seawater (ultrafiltered through a 1, 000 NMW filter). Figure 1 shows the partitioning results for Alon suspensions (0 - 5.0 mg 1-i) in colloid-free seawater with dissolved organic matter (DOM) concentrations of 0.1 mg C1-1 (Figure la) and 1 mg C1-1 (Figure ib). Separation of the samples into dissolved (< 1, 000 NMW), colloidal (1, 000 NMW-0.2 pm), and particulate (> 0.2 pm) fractions showed that with increasing Alon concentration, colloidal Th (The) decreased concurrently with an increase in particulate Th (Thy ). Alon concentration had little effect on the percentage of Th in the dissolved fraction (Thd). The colloidal fraction of 234 Th was found, by sequential filtrations, to pass ultra-filters with nominal molecular weight cut-offs of > 5, 000 and to be retained by the YM2 ultrafilter (1, 000 NMW). Under the conditions of Figure 1 (i. e. colloid-free seawa-ter), hydrolysis complexes and organic complexes are the only species of Th present in solution (13). Therefore, the colloidal 234 Th measured in the photo-oxidized seawater samples (Figure la) was assumed to consist of Th hydroxide complexes. The colloidal Th in the 1.0 mg C1-1 DOM samples (Figure lb) was assumed to be predominantly bound in hydroxides as well since < 5% of the total sample 14C was detected in the colloidal fraction. This concentration of Phaeodactylum tricornutum exudates (< 0.05 mg C1-1 ) would complex < 3% of the total Th in the sample (7). Amicon YM2 ul-trafilters have been reported to retain species with molecular weights much lower than their nominal molecular weight cut-off of 1, 000 (down to 200 MW) and to have a 50% retention at 380 MW (18). Therefore, these hydroxide complexes may be Th(OH)4, the dominant species of the Th in seawater (13), or polynuclear hydroxide complexes. The dissolved Th species in Figure 1 are probably organic complexes or hydrolysis complexes not retained by the YM2 ultrafilter. Phaeodactylum tricornutum exudates have been found to form strong complexes with Th; at 1.0 mg C 1 -1, 47% of total Th is bound in dissolved organic complexes (7). From a study of Th adsorption by metal oxides in seawater, Hunter et al. (19)
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