Article pubs.acs.org/IECR
Design of a Laboratory Method for Rapid Evaluation of Experimental Flocculants Rafael A. Garcia,* Stephanie A. Riner,† and George J. Piazza Biobased and Other Animal Co-products Research Unit, Eastern Regional Research Center, Agricultural Research Service, United States Department of Agriculture, 600 East Mermaid Lane, Wyndmoor, Pennsylvania 19038, United States S Supporting Information *
ABSTRACT: Reports of novel organic polymeric flocculants have become commonplace. The method used to test the effectiveness of these flocculants is most often the flocculation of a kaolin suspension in a jar test. The widely varying versions of this method that appear in the literature suffer from a range of weaknesses. The present research uses well-defined kaolin and confines testing to conditions in which the kaolin suspension is stable in the absence of a flocculant. The research examines all aspects of the conduct of the method, including clay dosing, mixing, settling time, and measurement to improve the sensitivity, reproducibility, and robustness of the method, and takes steps to avoid pitfalls that can reduce the validity of the method. Innovations include careful selection of the buffer system and instrument characteristics. Kaolin Clarification Effectiveness is introduced as a metric that gives a meaningful indication of the relative value of a novel flocculant while emphasizing the critical importance of test conditions. Together, the results form a set of recommended test conditions that should be useful for new flocculant research.
1. INTRODUCTION Flocculants are used in a wide variety of industrial and agricultural applications to promote the aggregation and settling of suspended particles. There is considerable research going into the development of new flocculants, especially biobased polymeric flocculants.1 New flocculants are often tested with aqueous suspensions of kaolin,2 a fine clay material that settles very slowly when suspended in water, and some variation on a procedure known as the jar test. The jar test is a simple method that was developed for use at water treatment facilities. It is intended to assist operators in optimizing the dosing of flocculants added to the water flowing through a particular facility in a particular time period. It involves dividing a water sample into four to six 1 L beakers and adding different amounts or types of flocculant to each beaker. The jar test apparatus puts the beakers through a stirring program intended to mimic the flash mixing, flocculation, and settling unit operations of the specific treatment plant.3 The formation of flocs is observed, and the residual turbidity of the water is measured. Although often referred to as the standard jar test, the method is in no way standardized.4 This is acceptable because in the method’s intended use, there is no need for jar test results to be comparable between different facilities, or even necessarily through time at a particular facility. Further, in typical use, the four to six beakers in a jar test apparatus are adequate. In many cases, qualitative or semiquantitative results are satisfactory. In the literature on experimental flocculants, the jar test of the water treatment plant is frequently adapted even though it is poorly suited to laboratory research. Each research group prepares their suspensions and performs jar tests differently (see Supporting Information Table S1, which compares the methods reported in 13 papers published in 2011−2012), producing incomparable results. Only a small number of This article not subject to U.S. Copyright. Published 2013 by the American Chemical Society
conditions can be tested simultaneously. Although pH and temperature are well-known to affect flocculant behavior, these are typically uncontrolled. There is rarely any indication that sensitivity or robustness has been considered in developing the methods. Many of the methods will have their results skewed by the presence of color in the suspension or the experimental flocculant.5 The present study systematically improves upon the weaknesses of previously reported methods.
2. EXPERIMENTAL SECTION Aluminum sulfate, MES, L-malic acid, Tris base, thimerosal, indigo carmine (FD&C Blue 2), tartrazine (FD&C Yellow 5), and Erythrosin B (FD&C Red 3) were obtained from SigmaAldrich (St. Louis, MO) and were of reagent grade or better. Chitosan and bovine hemoglobin were also obtained from Sigma-Aldrich, and described as ≥75% pure and lyophilized powder, respectively. A fine kaolin with the trade name Polygloss 90 produced by Huber Engineered Materials was donated by the M. F. Cachat Co. (Lakewood, OH). Anionic polyacrylamide with the trade name Superfloc A-110 was donated by Kemira (Atlanta, GA). Malic-MES-Tris (MMT) buffer was prepared according to the original paper describing it,6 with the exception that 0.01% (w/v) thimerosal was added as a preservative. This buffer is now commercially available from at least four manufacturers. These manufacturers describe buffer prepared in this manner as “1 M” or “nominal 1 M”. In fact, it is the molar concentration of the three buffer components combined that is approximately 1 M. To avoid confusion, we chose to follow this convention: 0.1 Received: Revised: Accepted: Published: 880
October 25, 2013 December 16, 2013 December 17, 2013 December 17, 2013 dx.doi.org/10.1021/ie4036115 | Ind. Eng. Chem. Res. 2014, 53, 880−886
Industrial & Engineering Chemistry Research
Article
M MMT will refer to a 1:10 dilution of the “1 M” stock solution, rather than to a true 0.1 M concentration of anything. Flocculant tests were carried out in 25 × 95 mm borosilicate glass vials. After 24 mL of kaolin suspension was added, turbidity measurements were made on each vial using a 2100AN IS laboratory turbidimeter from Hach (Loveland, CO); the vials were inserted into the turbidimeter and read directly through the glass without disturbing the suspension. In some experiments, absorbance was measured using an 8453 UV−visible spectrophotometer from Agilent Technologies (Santa Clara, CA). Immediately after the prescribed amount of flocculant was added to the vials, they were mixed using either a gyrotory shaker-model G2 (New Brunswick Scientific Co., Edison, NJ), a tube/vial rotator (Glas-Col, Terre Haute, IN), a Vortex-Genie 2 (Scientific Industries, Inc., Bohemia, NY), or by repeated manual inversion. Following the mixing, samples were degassed by placing the vial under partial vacuum until bubbles ceased appearing. Suspensions were allowed to flocculate and settle quiescently at 20 ± 1 °C. Turbidity was measured again at the prescribed end point of the test. All experimental treatments were replicated three or more times.
stock suspension over many days minimizes the effect of small variations in concentration each time a suspension is prepared. The stability of the suspensions was also found to be insensitive to pH in the range of pH 5 to at least 9 (Figure 2). At pH less than 5, the suspension is much less stable, presumably because the negative surface charge on particles is neutralized.
3. RESULTS AND DISCUSSION 3.1. Kaolin Suspensions. For an assay in which the flocculant is the object of study, rather than the “wastewater”, it is advantageous to use a wastewater that is very stable, so that changes observed when adding the flocculant are all attributable to the action of the flocculant and not confounded with changes that the wastewater would experience in the absence of the flocculant. In this sense, the kaolin suspensions used in the present study are well suited to such an assay. The turbidity of an unagitated suspension is essentially constant over the period of time relevant for a convenient benchtop assay (Figure 1). Further, stock suspensions of kaolin remain usable for extended periods; we found that at a range of concentrations, stock suspensions could be resuspended to the initial turbidity for at least 6 weeks after preparation through vigorous mixing of the suspension in the stock bottle (data not shown). Use of a single
Figure 2. Turbidity of a kaolin suspension (initially 3 g/L) after settling for 22 h versus pH. No flocculant added. Data points represent the average of three replicate experiments; error bars representing ±1 standard deviation are present for all points, but most are obscured by the data point markers.
Use of kaolin for a semistandardized assay presents a problem because the range of products labeled as “kaolin” varies significantly in physical and chemical properties. Kaolin that is commercially available in laboratory quantities is typically not well characterized, and may vary between lots. Suppliers of kaolin for industrial purposes, on the other hand, can provide multiple varieties of consistent and wellcharacterized kaolin, but their products are not available for sale in small quantities. The variable characteristics that we believe to be relevant to this assay are particle size, as well as treatments that affect chemical composition, termed calcining or surface treating. The kaolin used here has an average particle diameter of 0.6 μm and is neither calcined nor surface treated. We suggest that researchers work with a local industrial kaolin supplier to find a similar product; it is our experience that small samples are often available for the cost of shipping. Because the behavior of suspensions and flocculants can be greatly influenced by pH, benchtop testing of new flocculants benefits from the use of a buffered suspension with a known and stable pH. The MMT buffer system, originally developed for protein crystallization studies,6 is advantageous for flocculant testing. It provides steady buffering in the entire range of pH 4−9. Stock solutions of pH 4 and pH 9 are easily combined to produce any desired pH in this range. Although simple to prepare, premixed stock solutions are commercially available. Most importantly, we selected this buffer system from a family of similar buffers because all of the components have low tendency to form complexes with multivalent cations.7,8 This feature limits the interference of the buffer with flocculation studies including inorganic flocculants such as
Figure 1. Change in turbidity of kaolin suspensions over the period of a typical work day. Data points represent the average of three replicate experiments; error bars representing ±1 standard deviation are present for all points, but most are obscured by the data point markers. 881
dx.doi.org/10.1021/ie4036115 | Ind. Eng. Chem. Res. 2014, 53, 880−886
Industrial & Engineering Chemistry Research
Article
key to the mechanism of many inorganic flocculants,9 so buffering against a pH change would be inappropriate. Kaolin suspensions buffered to pH 5.5 by 25 mM MMT and microbiologically stabilized with 0.01% thimerosal were adopted as the standard for this assay. 3.2. Kaolin Suspension Clarification Measurement. Kaolin suspension clarification assays must quantify the interference between light passing through the suspension and the suspension particles, before and after the addition of a flocculant. In the literature, initial suspension concentrations are most commonly correlated with light absorbance measured using a spectrophotometer, or light scattering using a single beam type turbidimeter. The correlation between initial clay concentration and absorbance is strong, but the linear range is very limited (Figure 4a). Four wavelengths that have been used in the literature for this purpose were tested; longer wavelengths had greater linear ranges, but none were useful above 1.5 g/L kaolin. This limitation can be overcome by dilution of the unknown sample, but it is hard to rule out the possibility that dilution and mixing will have unintended effects on the flocculation and settling behavior. Single beam turbidimeters are the most common type of turbidimeter; they measure the light scattered perpendicular to a beam of light passing through the sample. These types of instruments are well-suited for measuring the light scattered by low levels of suspended particles, but they are known to be inaccurate when the turbidity is greater than the amount that is produced by about 0.06 g/L kaolin (40 NTU).10 We did not include a turbidimeter of this type in the present research. A ratio turbidimeter is a more sophisticated type of turbidimeter that measures backscattered and forward-scattered light in addition to perpendicularly scattered light and light transmitted through the sample. The correlation between suspended kaolin concentration and turbidity measured by the ratio turbidimeter used in this study was very strong (R2 = 0.9915) over the entire range of 0−6 g/L kaolin (Figure 4b, dashed line). This high correlation coefficient, however, is somewhat deceptive; examination of the data on a log−log plot shows that the linear relationship does not hold at low kaolin concentrations (Figure 4c, dashed line). A power law relationship also fits the data well (R2 = 0.9973) and does
aluminum sulfate or the calcium ions involved in anionic PAM flocculation. Our addition of 0.01% thimerosal to the buffer inhibits microbial growth in the stock solutions. Excessively dilute buffer will not have the capacity to resist perturbations to the pH, while excessively concentrated buffer may have unintended effects on the suspension properties. To determine a minimum buffer concentration that will control pH adequately in most practical scenarios, very high loads of kaolin and a variety of flocculants were added to buffer in a range of pH and buffer concentrations (Table 1). The concentration of Table 1. Minimum Concentration of MMT Buffer Required To Prevent pH Change of >0.15 pH Units upon the Addition of High Loads of Various Additives, with Buffer Initially at pH 5, 7, or 9 minimum buffer conc. (mM) additive
mg/L
pH 5
pH 7
pH 9
PAM hemoglobin kaolin chitosan Al sulfate
20 200 6000 42 1000
1 3 9 27 243
1 1 3 27 243
1 3 1 81 243
each additive was greater than the highest concentration we could find cited in the water research literature. 27 mM MMT buffer limited the perturbation of the pH to less than 0.15 pH units, in most cases, when kaolin or the organic, polymeric flocculants (PAM, chitosan, and hemoglobin) were added. Aluminum sulfate, however, overwhelmed the buffering capacity unless the buffer was much more concentrated. At buffer concentrations up to at least 27 mM, kaolin suspension clarification is unaffected by pH, in the range of at least pH 5−9 (Figure 3a). At greater buffer concentrations, the buffer causes increased suspension clarification rates (Figure 3b); this is undesirable because it would interfere with measurement of changes in turbidity due to the addition of a flocculant. Because of the destabilization of the suspension at high buffer concentrations, this system is not well suited for use with many inorganic flocculants. Additionally, pH reduction is
Figure 3. The effect of MES buffer pH and concentration on the stability of kaolin suspensions. In (a), the buffer concentration is held constant at 27 mM while the pH is varied; in (b), the pH is held constant at 5 while the buffer concentration is varied. Standard deviations for all data points are small, and error bars are omitted from the figure for clarity. 882
dx.doi.org/10.1021/ie4036115 | Ind. Eng. Chem. Res. 2014, 53, 880−886
Industrial & Engineering Chemistry Research
Article
Figure 4. Concentration of suspended kaolin versus instrument response. (A) Light absorbance at a range of wavelengths measured using a spectrophotometer, (B) turbidity measured using a ratio turbidimeter plotted on linear scales, and (C) the same data plotted on log10 scales. In both (B) and (C), the dashed line represents a best-fit linear regression equation, and the solid line represents a best fit power law equation. Error bars representing ±1 standard deviation are present for all points, but most are obscured by the data point markers.
not show the tendency to deviate at low concentrations (Figure 4b and c, solid lines). Colored substances dissolved in the water or the flocculant itself may interfere with spectrophotometer or turbidimeter measurements by absorbing light, leading to inaccurate results. This problem is minimized in ratio turbidimeters; the effect of light absorption appears in both the numerator and the denominator of the equation used to calculate turbidity and is mathematically canceled out.10 Separately, most turbidimeters sold in the United States (both single beam and ratio) use a polychromatic visible light source to conform to EPA method 180.1; worldwide, turbidimeters more commonly use a monochromatic infrared light (IR) source to conform to international standard ISO 7027. Colored substances tend to have low absorbance at the IR wavelength used (870 nm). In the present study, a turbidimeter with an IR light source was used to take advantage of this property. To examine the magnitude of advantage provided by these instrumentation selections, dyes with absorbance maxima in three different parts of the visible spectrum were added to buffers and kaolin suspensions (Table 2). The differences in absorbance or turbidity due to the presence of the dyes were translated into kaolin concentrations using standard curves. All of the dyes had little influence on the estimates of initial kaolin concentration generated from turbidimeter measurements (
