Environ. Sci. Technol. 1988, 20, 155-160
for a given system may be well-predicted based on the knowledge of chromate dissociation equilibria and the ionic strength of the cooling water.
Glossary molar concentration of species i in the aqueous [il phase molar concentration of species i in the exchanger El phase activity of species i in the aqueous phase, mequiv/L activity of species i in the exchanger phase, mequiv/g or mequiv/mL activity coefficient of species i in the aqueous phase Yi activity coefficient of species i in exchanger phase fi concentration of species i in aqueous phase, meCi quiv/L or mg/L concentration of species i in exchanger phase, ci mequiv/g or mequiv/mL K thermodynamic equilbrium constant total liquid-phase concentration, mequiv/L resin exchange capacity, mequiv/mL or mequiv/g equivalent fraction of species i in the liquid phase xi equivalent fraction of species i in the exchanger Yi phase separation factor of i with respect to j, dimension“ij less distribution coefficient of species i Xi t time, s or h I ionic strength, mol/L Cr(V1) total hexavalent chromium concentration, mg/L Abbreviations *R asterisk denotes exchanger phase SBA strongly basic anion resin WBA weakly basic anion resin BV bed volumes SLV superficial linear velocity (Re) particle Reynolds number EB8T empty bed contact time
1
Registry No. IRA-900, 9050-97-9; IRA-94, 39409-19-3.
Literature Cited Yamamoto,D.; Koichi, Y.; Osamu, A. “proceedings,Cooling Tower Institute Annual Meeting”; Houston, TX, 1975. Kunin, R. “Amber Hi-Lites No. 151”;Rohm and Haas Co.: Philadelphia, May 1976. Newman, J.; Reed, L. “Proceedings,Water-1979”;AIChE, 1980; Vol. 197, no. 76. Richardson, E.; Stobbe, E.; Bernstein,S. Enuiron. Sc. and Technol. 1968,2, 1006.
Butler, J. N. “IonicEquilibrium”;Addison-Wesley: New York, 1967. Tong, J. Y.; King, E. L. J.Am. Chem. SOC.1953,75,6180. Arden, T. V.; Giddings, M. J. Appl. Chem. 1961,11,229. Sengupta, A. K., Ph.D. Dissertation, University of Houston-University Park, Houston, TX, 1984. Sengupta, A. K,; Clifford,D. Ind. Eng. Chem. Fundam., in press. Sengupta, A. K.; Clifford, D. Reactive Polymers, Ion Exchangers, Sorbents, 1985, in press. APHA-AWAWA-WPCF“Standard Methods for the Examination of Water and Wastewater”;Washington, D.C., 1980.
Reichenberg, D.; McCauley, D. J. J. Chem. SOC.1955, 2741-2749.
Helfferich,F. “Ion Exchange”;Xerox University Microfilms: Ann Arbor, MI, 1961. Myers, G. E.; Boyd, G. E. J . Phys. Chem. 1956,60, 521. Miller, W. S. “Ion Exchange for Pollution Control”;CRC: Boca Raton, FL, 1978; Vol. 1, p 191. Stumm, W.; Morgan, J. “Aquatic Chemistry”;Wiley: New York, 1981. Rohm and Haas Co. “Amber-Hi-Lites”;1978; no. 159. Clifford, D.; Weber, Walter Reactive Polymers, Ion Exchangers, Sorbents 1983, I , 77. Received for review April 5, 1985. Accepted August 13, 1985.
Comparison of Methods for the Concentration of Suspended Sediment in River Water for Subsequent Chemical Analysis Arthur J. Horowltz US. Geological Survey, 6481-H Peachtree Industrial Blvd., Doraville, Georgia 30340
rn Centrifugation, settling/centrifugation, and backflushfiltration procedures have been tested for the concentration of suspended sediment from water for subsequent tracemetal analysis. Either of the first two procedures is comparable with in-line filtration and can be carried out precisely, accurately, and with a facility that makes the procedures amenable to large-scale sampling and analysis programs. There is less potential for post-sampling alteration of suspended sediment-associated metal concentrations with the centrifugation procedure because sample stabilization is accomplished more rapidly than with settling/centrifugation. Sample preservation can be achieved by chilling. Suspended sediment associated metal levels can best be determined by direct analysis but can also be estimated from the difference between a set of unfiltered-digested and filtered subsamples. However, when suspended sediment concentrations ( a 5 0 mg/L) or trace-metal levels are low, the direct analysis approach makes quantitation more accurate and precise and can be accomdished with simder analvtical mocedures. Introduction Suspended sediment plays an extremely important role in the transport and geochemical cycling of trace metals
in aquatic systems (1-9). In addition, the sampling and subsequent chemical analysis of this material have been used to locate ore deposits, to identify long-term trends in water quality, and to identify sources of anthropogenic pollution (2,5-12). The classical procedure for collecting and concentrating suspended sediments entails in-line filtration using preweighed 0.45-pm membrane filters; usually two are loaded with the upper one serving to collect the sample and the lower one acting as a procedural and analytical blank. This labor-intensive and costly method is probably the most commonly used procedure for the concentration of suspended sediments for subsequent chemical analysis and is usually the method of choice for studies where sample numbers are limited (e.g., 13). However, if large-scale studies are undertaken, with numerous sampling sites or high sampling frequencies or both, such as the U.S. Geological Survey’s (USGS) National Stream Quality Accounting Network (NASQAN), where some 500 sites are sampled 4-6 times a year (14)) the analytical costs and manpower demands of the in-line fiitration procedure can become prohibitive. It is estimated that it would require a t least two full-time employees just to weigh, load, and supply users with in-line filter holders. Also, extra work and care is needed to unload the sedi-
Not subject to U.S. Copyright. Publlshed 1986 by the American Chemical Society
Environ. Sci. Technoi., Vol. 20, No. 2, 1986
155
ment-laden filters and to dry and reweigh them to determine sample weights. Additionally, the presence of the filter pad itself increases the time and difficulty involved in digesting the sample for subsequent chemical analysis, and for accurate work, each sample plus pad must be matched with an additional pad to serve as a procedural and analytical blank. Finally, when suspended sediment concentrations are low, very fine-grained, or concentrations of algae or organic matter are high, the filtration of adequate volumes of water can take hours (e.g., 13; this study, where 1-L aliquots of both Skunk River and Suwanee Creek water took over 6 h to filter). At the present time, many laboratories avoid filtration problems by calculating suspended sediment-trace metal concentrations by difference. A water sample is collected, and an aliquot is filtered through a 0.45-pm membrane filter. An unfiltered aliquot from the same sample is digested with a combination of weak acid (e.g., 10% HC1) and heat and then filtered to remove undigested solids (15). The digested and the undigested field-filtered samples are then analyzed for a range of trace metals. The concentration of sediment-associated trace metals is calculated from the difference between the analytical results of the two determinations. Because of the low metal levels encountered when suspended sediment concentrations are small, as well as the problems associated with determining metal concentrations by difference, it has long been felt that this procedure lacks sufficient precision and accuracy. Further, a direct analysis would reduce analytical costs since metal concentrations would be higher, and thus, simpler quantitation techniques could be employed (e.g., elimination of chelation extraction, use of graphite furnace). Finally, if only suspended sediment associated metals were of interest, only one determination would be required instead of two. This latter point is applicable only if the in-line filtration procedure is not employed since it requires a sample/blank pair for each determination. On the basis of the foregoing, alternative methods to the in-line filtration procedure, for use in the concentration of suspended sediment for subsequent direct chemical analysis, needed to be examined for potential use. Alternatives needed to meet certain criteria prior to field and laboratory trials include the following: (1)Existing sampling methods and procedures, such as depth-integrated sampling (16), should be retained. (2) The alternatives should produce at least comparable chemical data to those obtained with the in-line filtration method based upon the analytical methods employed for metal quantitation. (3) Sufficient material should be collected (about 100 mg on a dry-weight basis) for metal quantitation to be accomplished by relatively simple analytical procedures such as an acid digestion and analysis by flame atomic absorption spectroscopy without the use of chemical preconcentration steps. (4) The procedure should be less labor intensive than in-line filtration and amenable to large-scale programs. (5) The method should be usable under adverse field conditions such that processing could take place in a "nonlaboratory" environment. Subsequent quantitation could then take place in a laboratory, typically at a distance from either the sampling site or the processing locale and several days after collection/sample processing/sample stabilization.
Experimental Section Proposed Concentration Procedures. Three potential alternative procedures met the criteria: centrifugation, 156
Environ. Sci. Technol., Vol. 20, No. 2, 1986
settling/centrifugation, and backflush-filtration. Centrifugation. Water is collected, mixed, stored in appropriate containers (e.g., 1-L acid-washed plastic bottles), and chilled on ice. Processing entails centrifugation with a fixed angle-head, or swing-bucket rotor, capable of holding about 2 L and speeds of at least 5000 rpm (-2560 X g ) to separate the solids from the supernate. Centrifugation is repeated until sufficient sediment is collected in a single centrifuge tube. The majority of the supernate in this tube is decanted; the tube is sealed and chilled and is then sent to a laboratory for final processing and analysis. This includes drying the sediment by a method consistent with the metals to be determined (in this study, oven drying at 105 "C), removing it from the tube, digestion, and chemical analysis. Settling/ Centrifugation, Water is collected, mixed, and stored as above, in 1-L acid-washed plastic bottles. According to Stokes law, a 0.45-pm particle will settle 10 mm in 12-13 h, at room temperature. After 7 days, 2 / 3 to 3/4 of the supernate in each bottle can be siphoned off, the remaining water and sediment combined into a single container, and shipped to a laboratory for analysis. Final processing entails centrifugation to remove the remaining supernate and to concentrate the sediment in a single tube. Further processing is as above, to obtain analytical results. Backflush-Filtration. Water is collected, mixed, and filtered through a 142-mm, 0.45-pm backflush filtration system (17) used in conjunction with a reversible peristaltic pump. After the water has been filtered, or when the filter begins to clog, the pump is reversed, and the collected sediment is backflushed off the filter pad into an appropriate container (e.g., a 1-L acid-washed plastic bottle). Processing continues until sufficient water has been filtered/backflushed. The final concentrate, which normally does not exceed 1 L, is chilled and sent to an analytical laboratory for final processing and quantitation as described in the centrifugation section above. A number of other procedures were considered and rejected, because of prohibitive cost and/or labor intensity. For example, evaporation of a whole water sample by heating or freeze-drying was eliminated because of the large volumes of water involved and the associated costs of shipping, handling, and storage. Test Site Selection. The three proposed alterhative procedures, as well as the in-line filtration method, which served as a benchmark against which the alternatives were compared, were tested a t four separate locations, which represented significantly different hydrologic settings and suspended sediment loads. The sites and suspended sediment concentrations were the Skunk River near Augusta, IA (75 mg/L), the Suwanee Creek near Duluth, GA (154 mg/L), the Mississippi River at Baton Rouge, LA (228 mg/L), and the Susquehanna River at Harrisburg, PA (575 mg/L). Site selection was also based upon historical suspended sediment concentrations. Suspended loads had to be sufficiently high to permit adequate sample quantity to test the four methods on material that came from a single full churn splitter (approximately a 12-L working capacity, Bel Art 84013oooO) to reduce the variability likely from multiple sampling runs. All but the Susquehanna River sample are natural; this sample is a construct made from mixing fine-grained bottom sediment and river water in a churn splitter to create a sample with a high suspended sediment load. Sample Processing. Depth-integrated whole water samples were collected a t each site according to standard procedures (15) and composited in the churn splitter. Twelve 1-L aliquots were withdrawn from the splitter, plus
Table I. Comparison of Chemical Concentrations Obtained by Various Suspended Sediment Concentration Techniques for Four Sample Sites mg/k Ni
wt. %
%
A1
Ti
recovery"
0.21 0.22 0.21 0.19
5.1 5.0 5.2 4.2
0.24 0.24 0.25 0.19
91 84 30
Suwanee Creek near Duluth, GA (154 mg/L) 33 17 2.2 75 5.0 35 16 2.2 73 5.1 34 17 2.1 72 5.0 41 21 2.6 83 6.0
0.17 0.18 0.18 0.20
12.8 12.9 12.7 13.9
0.53 0.54 0.53 0.59
96 96 33
106 104 102 100
Mississippi River at Baton Rouge, LA (228 mg/L) 36 11 3.3 61 3.9 36 10 3.0 58 3.9 36 10 2.9 62 3.8 38 10 3.0 60 3.9
0.14 0.13 0.13 0.14
7.8 7.9 7.8 7.9
0.36 0.34 0.35 0.35
90 87 65
234 236 232 410
39 38 36 65
Susquehanna River at Harrisburg, PA (575 mg/L) 52 0.8 44 3.4 78 44 3.2 53 0.8 76 75 52 0.7 43 3.1 63 5.0 105 85 4.5
0.18 0.18 0.17 0.30
4.2 4.2 4.4 7.5
0.34 0.34 0.33 0.35
96 96 16
f 2
f4
method
Cu
Zn
Pb
in-line centrifugation settling backflushing
66 68 65 61
224 225 226 202
113 106 116 100
Skunk River near Augusta, IA (75 mg/L) 33 9 7.2 51 2.2 29 10 7.1 49 2.4 11 6.7 49 2.3 29 25 10 6.3 40 1.7
in-line centrifugation settling backflushing
36 34 35 38
181 180 182 212
44 44 44 51
in-line centrifugation settling backflushing
36 37 37 38
167 172 170 172
in-line centrifugation settling backflushing
39 38 36 70
precisionb
f5
f6
Co
f7
Cd
f17
Cr
f5
Fe
f3
Mn
f10
f2
-
-
-
-
14
'Recovery-the amount of sediment available for chemical analysis (e.g., what could be removed from the centrifuge tube), not the total amount of sediment in the tube itself. The percentage is based upon the estimated sediment available in the sample from the sediment concentration determination and the quantity of water processed. Precision (%)-based upon the concurrent analysis of NBS Estuarine Sediment and USGS standards W-2, G-2, MAG-1, BCR-1, and SGR-1.
a 100-mL aliquot for determining the suspended sediment concentration. The bottles were labeled, chilled, and returned to the local USGS District Office. In-line filtration and the backflush-filtration procedure to the point of obtaining a concentrate were carried out on the date of sampling (processing began within 2-4 h). The concentrate, the in-line filters in their holder, and the remaining sample bottles were kept chilled and sent to the USGS Atlanta laboratory for the centrifugation and settling/ centrifugation procedures, as well as for trace-metal quantitation. Shipment took from 3 to 5 days. Centrifugation and processing of the backflush-filtration concentrate was carried out on the day of arrival. At the same time, the in-line filter holder was opened and the two preweighed filter pads removed and stored in a desiccator to dry to constant weight. The sample aliquots destined for the settling/centrifugation procedure were thoroughly shaken and stored on a shelf for 7 days. At the end of that time, the supernate was siphoned by water aspiration through a J-tube, the remainder composited, centrifuged, all sediment collected in one tube, and dried. Throughout sample processing, whenever centrifugation was employed, the same conditions were maintained. The centrifuge had a six-place fixed angle-head rotor capable of processing approximately 1900 mL a t a time. Based upon calculations, and the particular conditions for the centrifuge, a 0.45-pm particle would take a little more than 1min to settle a t 10 000 rpm ( 10250 X g at the center of the tube), ignoring the controversial turbidity current gradient theory proposed by Jackson (18). With startup, and automatic braking to prevent particle resuspension, decantation, and transfer of sediment into a single tube, it was possible to process 5 L of whole water, concentrate, etc. in about 1h. Sediment transfer into a single tube was carried out with a plastic wash bottle filled with the supernate decanted from several of the tubes. Analytical Methods. The dried and weighed suspended sediment samples were digested according to the N
procedures of Horowitz and Elrick (19) with HF/ HC10,/HN03. Briefly, the sample is digested first with "OB, then twice with HF/HC104, and finally with HC104 alone, in open 100-mL beakers made of Teflon on a hotplate a t 200 OC. In-line filter samples and pads, and blank pads, were digested in a single beaker; no attempt was made to remove the sample from the sample pad. Final solutions were made to volume with 2% HC1. Quantitation was by flame atomic absorption spectrometry using mixed salt standards and, where appropriate, deuterium source background correction using a Varian Model AA 975. A t the levels encountered in these samples, precision, based upon concurrent replicate analyses of National Bureau of Standards (NBS) sediment and USGS rock standards were generally better than *5%, with the exception of Mn (f10%) and Cd (f15%) (Table I). No significant bias was observed. Due to the small amounts of sample available and the need to split it for separate procedures, replicate sample analyses were not feasible. However, the good comparability between the in-line, Centrifugation,and settling/centrifugation procedures and the results for the standards indicates that the analytical results are accurate and precise within the reported limits (Table I). The total recoverable and dissolved methods used to calculate suspended sediment trace metals by difference, used for comparison with the Susquehanna River and Suwanee Creek samples, are described in Skougstad et al. (15))and the determination of suspended sediment concentration was done following the procedures of Guy (20).
Results and Discussion Comparison with the In-Line Method. The analytical results for the in-line filtration procedure, as well as for the three alternative methods, for the four sample sites are presented in Table I. Examination of the data indicates that when contrasted with the in-line method, centrifugation and settling/centrifugation provide comEnviron. Sci. Technol., Vol. 20, No. 2, 1986
157
Table 11. Comparison of Direct Total Analysis with Indirect Total Recoverable Analysis of Suspended Sediment (Concentrations in pg/L) type of analysis
Cu
Zn
Pb
Ni
Co
Cd
Cr
Fe
tot direct" tot recov. calcdb
22 24
135 126
Susquehanna River at Harrisburg, PA (575 mg/L) 22 44 30 0.44 25 18600 31 35 34
