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Effects of Humic Substances on Particle Formation, Growth, and Removal During Coagulation Gary L . Amy, Michael R. Collins, C. James Kuo, and Zaid K. Chowdhury Environmental Engineering Program, Department of Civil Engineering, University of Arizona, Tucson, AZ 85721 Roger C. Bales Department of Hydrology and Water Resources, University of Arizona, Tucson, AZ 85721
This chapter describes research examining (1) the effect of natural organic matter(NOM)on the formation, growth, and removal of particles during coagulation using aluminum sulfate and (2) the effect of particles on NOM removal. Several sources of NOM and several types of mineral particles were studied under water treatment con ditions. Particle formation, growth, and removal were found to be significantly affected by initial particletypeand concentration, NOM type and concentration,pH,and aluminum sulfate dose.
SURFACE WATER CONTAN IS PARTICLES
ranging in size from submicrometer to supramicrometer dimensions, as well as aquatic natural organic matter (NOM), including humic substances. Important objectives of both conven tional surface-water treatment and direct filtration are removal of both tur bidity-causing particles and Ν Ο Μ . Total particle number is reduced during coagulation / flocculation by inducing particle growth, but subsequent sol ids-liquid separation (i.e., sedimentation and filtration) removes floe com posed of aggregated particles and precipitated aluminum hydroxide. 0065-2393/89/0219-0443$06.00/0 © 1989 American Chemical Society
In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.
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AQUATIC HUMIC SUBSTANCES
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Experimental Design Four independent series of experiments (A through D) were conducted with solutions containing a model particle and either aquatic dissolved organic matter ( D O M ) or soil fulvic acid. A l l experiments evaluated aluminum sulfate coagulation by using a conventional jar test apparatus, but mixing conditions (Table I) and "postmixing" sample processing were different. The principal objective of series A experiments was to evaluate particle removal by passing flocculated water through a laboratory sand filter functioning as a batchmode direct-filtration apparatus. The dimensions and operating conditions of the laboratory sand filter are described elsewhere (J). In contrast, the major objective of series Β through D experiments was to study particle formation and growth. The filtrate was characterized in series A . In series B - D , the flocculated suspension was analyzed. For each series, a preliminary set of experiments was run to provide a basis for selecting aluminum doses. The organic substances examined were a soil-derived fulvic acid (series A and D) and D O M isolated from two surface waters: the Grasse River in New York (series B) and the Edisto River in South Carolina (series C). D O M was defined as organic matter passing a 0.45-μπι membrane filter. Two parameters used for particle characterization were the total particle number (TPN), which represents the summation of particle concentrations observed in all channels of the particle counter, and the volume average diameter (d ), which is a volume-weighted average. T P N was used to indicate overall particle removal, and d indicated particle-size changes. Although values of T P N and d for the initial conditions reflect only the model particle, these same parameters for the final water reflect aluminum-particle-humic aggregates in the filtered water (series A) or in the flocculated suspension (B through D). v
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Experimental Details The hydrophobic (humic) fraction of the DOM, which was used in the experiments, was isolated by adsorption onto resin (XAD-8, Rohm and Haas), according to the method of Thurman and Malcolm (2). The average molecular weight of the DOM was determined by ultrafiltration (3). Carboxylic acidity was measured by potentiometric titration of the hydrophobic fraction from pH 3.0 to 8.0 (4). Nonpurgeable organic carbon (NPOC) was determined with an organic carbon analyzer (Dorhmann DC-80), and UV absorbance was measured with a UV-visible spectro photometer (Perkin-Elmer Model 200). Three different particles were studied: (1) kaolinite clay that contained significant amounts of submicrometer material, series A; (2) Min-U-Sil-5 (Pennsylvania Glass Sand Corporation), series Β and C; and (3) Min-U-Sil-15, series D. The Min-U-Sil materials represent crystaline silica, Si0 . Particle enumeration and size character ization were done with an optical particle counter (Hiac 4100, Pacific Scientific Inc.), using a 2.5-150-μπι sensor (series A and B) and a l-60-μπι sensor (series C and D). The more sensitive sensor was acquired midway through the research. 2
In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.
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Electrophoretic mobilities were measured on the coagulant suspensions with a microelectrophoresis apparatus (Rank Brothers Mark II). An equilibration time of 12 h was used for DOM-coating of particles.
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Results The Grasse River contained slightly more D O M than the Edisto River (Table II). Average NPOC-based molecular weights for the two rivers were com parable, although that for the fulvic acid was much higher. Carboxylic acidities differed by a factor of 2, but are comparable to values reported in the literature. The characteristics summarized in Table II for the natural waters reflect the original water, as well as actual experimental conditions. The M i n - U - S i l - 5 (Si0 ) had average d values of 4.2 and 3.4 μηα, using the 2.5-150 and l - 6 0 - μ π ι sensors, respectively. Addition of the lower range sensor midway through the experiments provided more information on smaller-particle removal. The d for M i n - U - S i l - 1 5 (Si0 ) was 6.3 μπι, using the 1-60-μπι sensor. Electrophoretic mobilities of the model particles as a function of p H are shown in Figure 1. Without the presence of humic substances and D O M , mobilities for S i 0 and kaolinite ranged from positive to negative, with observed p H values at the isoelectric point (pH ) of 2.6 and 4.6, respec tively. The DOM-coated silica exhibited a more negative mobility than the uncoated silica over the p H range of about 4.0 to 7.0, while the mobility of the DOM-coated kaolinite remained negative over the p H range of about 3.0 to 6.0. The effects of D O M on mobility were more pronounced for kaolinite than silica. Also shown in Figure 1 are mobilities for Al(OH) (s) formed under conditions of homogeneous nucleation, indicating that the aluminum hydroxide precipitate formed can exhibit a positive or negative charge with a p H ^ of about 6.5. The statistical significance of observed differences was evaluated by using the Duncan's multiple range (DMR) test, which has inherent advan tages over the more commonly used f-test when a number of comparisons are made within a data set (5). The D M R test also permits a statistical comparison of pairs of parameter values. The data shown in Table I were subjected to a statistical analysis by the D M R test (6-8). Comparison of initial and final water characteristics shows an increase in d , a decrease in T P N , and a decrease in N P O C for most experiments (Table I). The following specific data comparisons are based on statistically significant differences in parameter levels at the 95% confidence level. In Series A , fulvic acid more adversely affected particle removal at the lower p H . Higher fulvic acid concentrations generally resulted in higher T P N levels in the filtrate at p H 5.5; at a higher p H of 8.5, no statistically significant differences were found for T P N levels in experiments with higher versus lower initial fulvic acid levels. This finding may be due to the greater 2
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2
2
iep
3
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In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.
In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.
B-l B-2 B-3 B-4 B-5 B-6 B-7
Exp. ID A4 A-2 A-3 A-4 A-5 A-6 A-7 A-8
none silica silica none silica silica silica
Grasse Grasse Grasse Grasse Grasse Grasse Grasse
II II II II II II II
a
7.0 7.0 7.0 8.5 8.5 5.5 5.5 0 5 10 0 10 5 5 2.0 2.0 2.0 2.0 2.0 2.0 4.0
1
210 140,000 290,000 210 290,000 140,000 140,000
b
4.2 4.2 — 4.2 4.2 4.2
v
b
5.0 5.0 5.0 5.0 5.0 5.0 5.0
930 10,000 5,100 70 5,700 3,500 6,200
1
Table I. Summary of Initial and Operational Conditions for Series A, B, C , and D Experiments Initial and Operating Conditions Initial Water Turb. Al TPN TPN Particle Organic Mixing d NPOC (mg/L) (mL- ) Cond. pH (NTU) (mh- ) Source (μτη) (mg/L) Type — 2.0 1,100 fulvic 5.5 0 1.9 90 none I 2.0 14,000 kaolin I 5.5 10 4.4 1.9 170 fulvic 4.0 1,100 none 8.5 0 — 1.9 90 fulvic I kaolin I 10 4.0 14,000 4.4 1.9 80 fulvic 8.5 2.0 5.5 0 1,100 — 7.7 200 none fulvic I 2.0 kaolin fulvic I 5.5 10 14,000 4.4 7.7 230 none 8.5 0 4.0 1,100 — 7.7 130 fulvic I 4.0 kaolin I 8.5 10 14,000 4.4 7.7 110 fulvic
Downloaded by UCSF LIB CKM RSCS MGMT on September 4, 2014 | http://pubs.acs.org Publication Date: December 15, 1988 | doi: 10.1021/ba-1988-0219.ch027
6.5 8.8 1.2 8.5 10.7 18.5 14.9
v
2.8 3.0 2.7 3.7 3.4 2.0 1.7
Final Water NPOC d (mg/L) (μτη) 1.0 4.9 1.1 4.6 0.9 5.1 0.8 5.4 3.4 4.8 3.5 4.7 3.0 5.0 3.3 5.0
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