INDUSTRIAL AND ENGINEERING CHEMISTRY
94
evidence of any jelly-like condition as caused by pectase. There was, however, a slight mold-like growth in the bottom of the reaction flask in the case of Protozyme Coarse in every experiment. This growth appeared in the flasks containing the other preparations as the pH was raised to 3.8, which appeared to be the optimum for this condition. The slight I
I
3.25
3.5
I
I
;fi
I
I
I
I
, 5.0
4.5
The data obtained here show that the presence of these diastatic preparations has a detrimental effect on the viscosity of a pectin solution, at.40’ C., in the pH range covered. The decrease in viscosity is most severe in the first half-hour of the reaction and is magnified as the hydrogen-ion concentration is buffered to a point where the diastatic efficiency of the preparation is increased. At the higher hydrogen-ion concentrations the diastatic action of the preparation may be greatly reduced, but the action of the pectolytic enzymes is practically absent. It is evident that we should carry out the hydrolysis a t these lower pH values which allow only a minimum percentage loss in jelly grade against losses approaching 50 per cent for a pH of 5.0, which is the pH generally recommended. The preparation of enzymes active a t high hydrogen-ion concentrations would evidently be desirable, in that the pectinase present would be less active. It might be possible to acclimatize the producing organism, such as Aspergillus oTyzae, to growth a t a low pH in order to discourage the formation of pectinase.
FIGURE4, VISCOSITYOF PECTINAFTER 30MINUTEACTIVITY OF DIASTATIC PREPARATIONS AT VARIOUS PH V.4LUES
growth always appeared a t the bottom of the flask and did not interfere with the sampling of the solution for the viscosity determination. TABLE11. JELLYGRADEOF PECTINAFTER TREATMENT WITH DIASTATIC PREPARATIONS FOR 30 MINUTES AT VARIOUSPH VALUES PH
VALUE 3.3 3.5 3.8 4.6 5.0
O.M% 0.42% 1.00% TAKA0.12% DIASTABE PROTODIABTASECLARASE P2 ZYME E m . 265 275 270 250 235 265 260 265 275 265 230 275 225 225 210 235 136 180 180 105
0.25% CHECK 290
...
285 270 235
Vol. 27, No. 1
LITERATURE CITED (1) Bourquelot and Herissey, Compt. rend., 127,191 (1898). (2) Ehrlich, F.,Biochem. Z., 250,524 (1932). (3) Gortner, R. A., “Outlines of Biochemistry,” p. 594, New Y o r k , John Wiley & Sons, 1929. (4) Jones, L. R., N. Y. Agr. Expt. Sta., Tech. Bull. 11 (1909). (5) Mehlitz, Alfred, Biochem. Z., 221,217 (1930). (6) Myers, P. B., and Baker, G. L., Del. Agr. Expt. Sta., Bull. 149 (1927). (7)Ibid., 187 (1934). (8) Waksman and Davison, “Enzymes,” p. 152,Baltimore, Williams & Wilkins Co., 1926. (9) Willaman, J. J., Minnesota Studies in Bid. Sci., 6,333(1927). RECEIVED September 25, 1934. Presented before the Division of Agricultural and Food Chemistry a t the 88th Meeting of the American Chemical Society, Cleveland, Ohio, September 10 to 14, 1934. Approved by the Director, Delaware Agricultural Experiment Station.
Coagulation of Soil Suspensions by Aluminum and Iron Salts Electrophoretic Study LLOYDR. SETTERAND SANTEMATTSON, Agricultural Experiment Station, New Brunswick, N. J.
T
H E purpose of the coagulation of waters with aluminum or iron salts is to facilitate the ultimate removal of suspended and colloidal material cornfaonly expressed as “turbidity.” Dilute solutions of alurmnum and iron salts under certain conditions form what is popularly called a “hydroxide floc.” On the apparent assumption that the iron or aluminum hydroxide floc is of primary importance, numerous investigations have been made dealing with the effect of various environmental factors on floc formation. Turbidity particles have long been known to be negatively charged, and one of the writers (6) has shown that sols of iron and aluminum, although amphoteric, are positively charged in neutral or acid solution. Obviously, therefore, the nature of negativity of the former is of considerable importance if the coagulation process consists of co-precipitation by the neutralization of charges. The object of this paper is to show, by electrophoretic measurements, that the composition and nature of turbidity particles (soil clays) have a material effect on the quantity of trivalent cations necessary for flocculation a t a given iso-
electric pH, and that the isoelectric pH can be varied a t will by the quantity of coagulant used and the proper pH adj ustment . Particles causing turbidities in streams are essentially soil washings (13); the quantity varies with the intensity and amount of rainfall. Robinson and co-workers (11) have shown great variations in the chemical composition of soils. The major differences appear to be: 1. The ratio of silica to the iron and aluminum oxide content. The sum of these substances constitutes roughly 80 per cent of the soil complex and is essentially the insoluble fraction. 2. The cation exchange capacity-i. e., that part of the calcium, magnesium, sodium, and otassium content which constitutes the ionizable and displacea%lefraction of the soil and imparts to the insoluble complex its negative charge. A direct relationship has been shown between the cation or base exchange capacity and the silica-sesquioxideratio (3,6).
EXPERIMENTAL PROCEDURE The influence of water turbidities subjected to chemical COagulation was made by selecting several soil colloids of vary-
January, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
ing silica-sesquioxide ratios, by replacing the exchangeable bases with sodium, and by treating the water suspensions with trivalent catio'n salts. The composition of the soil c o l l o i d s 1 I (la//", s e l e c t e d for study was reported b y Robinson (If) and Mattson (8). Their analyses are given in Table I. The differences of composition a r e shown particularly in the ratio of Si02 to R203. Values of 2.72, 1.89, and 0.31 w e r e reported f o r the Stockton, Sassafras, and Nipe soil colloids, respectively. A dilute i r o n o r aluminum salt solution (A) was prepared immediately b e f o re FIGURE 1. RELATION BETWEEN ELEC- use from a concenTROPHORETIC MIGRATION VELOCITIES trated stock solution AND PH FOR VARIOUSALUMINUM CHLO- of c. P. c h e m i c a l . RIDE TREATMENTS OF TYPICAL SOIL Solution B consisted COLLOIDS of one gram of electrodialyxed soil colloid dispersed in one liter of distilled water with the aid of a sufficient amount of sodium hydroxide to form a sodium-saturated complex. Equal volumes (20 ml.) of solutions A and B were made acid or alkaline to cover the significant pH range. The acid (hydrochloric) and alkali (sodium hydroxide) were added to solutions A and B, respectively, prior to mixing. The mixture was made up to a total volume of 50 ml. The two solutions were rapidly and thoroughly mixed by pouring from one beaker to the other five times in 5 seconds. The mixture mas then transferred to a 100-ml. stoppered Pyrex test tube. Approximately ten mixtures, acidified or alkalinized to varying degrees, were employed to cover the significant pH range for each trivalent cation treatment of a single soil colloid. Observations of the rate of flocculation were made during the fist 10 minutes. Electrophoretic migration velocities (4) and colorimetric pH determinations were made after 1 or 7 days. TABLE I. COMPOSITION OF SOILCOLLOIDS STOCKTON CLAYADOBE,
CALIF.
SASSAFRAS SILTLOAM (SUBSOIL), MD.
95
creased, the negative migration velocity decreases eventually to zero. A further decrease of p H reverses the migration velocity to the cathode; i. e., the particles become positively charged. In certain instances a given complex was stable or flocculation was very slow. The approximate change from stability to instability-i. e., immediate flocculation-is indicated in each curve. A broken line indicates stability and a solid line complete flocculation. The results on the Stockton colloid which has a high silica content show: (1) If a small quantity of aluminum chloride is added to the colloid, the resultant aluminum-soil complex will not become isoelectric at any pH. However, by the addition of sufficient acid, the hydrogen-ion concentration represses the negative charge sufficiently to cause partial coagulation. (2) If a larger quantity (0.5 millimole) of aluminum chloride is added and the pH is decreased, the resultant aluminum-soil complex will be isoelectric at a pH of 4.25. (3) Further additions of aluminum chloride (1.5 and 4.0 millimoles) raise the isoelectric pH of the aluminum-soil complex (5.7 and 6.85, respectively, for the above amounts). It can be assumed that additional increases of aluminum in the complex will ultimately raise the isoelectric point to approach that of pure alumina. One of the writers ( 2 ) reported an isoelectric pH of 8.1 for aluminum chloride plus sodium hydroxide. (4) Flocculation occurs before complete neutralization of the negative charge. ( 5 ) When the concentration of coagulant is high with respect to the colloid (4 millimoles aluminum chloride per gram Stockton colloid), the resultant aluminum-soil complex becomes stable on the acid side of the isoelectric point owing to a strong positive charge (pH 4.5 to 6.3). Increasing hydrochloric acid concentration eventually decreases the charge, causing colloidal instability at a pH below 4.3.
NIPE CLAY
ORIENTE, CUBA
2.7
FIGURE2. RELATIONBETWEEN QUANTITYOF ALUMINUM CHLORIDE REQUIRED TO PRECIPITATE SOILCOLLOIDS ISOELECTRICALLY AT A GIVENPH
Moles Si02 Moles AlzOs FezO:
+
2.72
1.89
0.31
RESULTS OF EXPERIMENTS The significant results of treating one gram of colloid with aluminum chloride (pH adjusted) are shown in Figure 1 by plotting the pH against the migration velocity of the particles. Each curve represents a definite proportion of coagulant to one gram of soil colloid in a total volume of 2500 ml. I n any one curve i t is shown that at high p H values a n aluminum-soil complex is negatively charged-i. e., high migration velocity toward the anode-and, as the p H is de-
Similar results were obtained on the Sassafras and Kipe colloids. Comparison of the aluminum chloride treatment of the three soils shows: (1) With equivalent quantities of aluminum chloride, the higher the original silica-sesquioxide ratio the lower is the isoelectric pH. ( 2 ) The quantity of coagulant necessary for coagulation at a given p H decreases with a decrease in the silica-sesquioxide ratio, These relationships are more clearly shown in Figure 2 by plotting the isoelectric p H of aluminum-soil complexes against the moles of coagulant. For convenience, the grams of aluminum cation per 100 grams of soil are also shown. COMP.4RISON
O F I R O N AND
ALUMINUM AS COAGULANTS
Samples of Stockton soil colloid were neutralized to p H 7.0 and precipitated by the separate addition of increasing quantities of aluminum and ferric chlorides. The pH deter-
INDUSTRIAL AND ENGINEERING CHEMISTRY
96
mination and electrophoretic measurements were made after one week. (See Table I1 and Figures 3 and 4.) The latter show that to decrease the n e g a t i v i t y of the soil to the equivalent of 1 p per second volt per c m . , 2.32 and 2.68 millie q u i v a l e n t s of aluminum chloride and ferric chloride were required, and t h e r e s u l t a n t pH millie~uiv.Coagulant was 4.9 and 4.02, FIGURE 3. RELATION BETWEEN respectively ; using EQUIVALENT QUANTITIES OF COAGULANT AND ELECTROPHORETIC MIGRA- 1.875 milliequivaTION VELOCITY OF COAGULANT lents of coagulant, (STOCKTON COLLOID COMPLEX) the resultant migration velocities were -2.2 and -1.9 for ferric chloride and aluminum chloride, respectively.
Vol. 27, No. 1
treatment a t relatively high pH values is similar to a polyvalent anion; i. e., the acidoid (essentially the silica anion) is usually in excess of the basoid (iron and aluminum oxide). I n cases where the basoid is high, as in the K p e colloid, its effect may overbalance the acidoid effect at low pH values. For example, the mere acidification of the Kipe colloid was sufficient to neutralize the negative charge. This observation can be explained by the substitution of the less dissociated hydrogen cation for the original sodium cation attached to the complex -Si-0 Na+ which would decrease the negativity, and the substitution of a more dissociated cloro OH(Cl-) anion for the hydroxyl in the complex =A1 which would increase the positivity. An increase in the positive character in parts of the complex micelle would promote the coalescence of neighboring negative particles. Ferric chloride decreased the negativity of Stockton colloid to a far lesser degree than an equivalent quantity of aluminum chloride, but produced a greater acid reaction. It was previously pointed out that, within limits, the lower the pH the less coagulant was necessary for flocculation. This anomaly is apparently due to the more rapid rate of hydrolysis of the ferric chloride; consequently, a greater hy-
+
TABLE11. COMPARISON OF IRONAND ALUMINUM AS COAGULANTS (20 ml. of solution Aa mixed with 30 ml. of Bb) FLOCCULATION COAQULANT Immediate 1 wk. ELECTROPEOREBIB Miltiequivalents r/eec.-volta/cm. pH AlClJ 1.876 Slow xx c -1.9 5.7 2.12 Rapid xx -1.5 5.2 2.5 Rapid xx -0.4 4.7
4.6 4.3 4.1 xx 4.0 a Solution A = varying quantities of aluminum and iron salts in 1000 ml. water. b Solution B = 1 gram Stockton colloid -I- 1.1 milliequivalenta NaOH in 1500 ml. water (pH 7.0) xx = oomplete,'x 'incomplete flocculation, FeCb
1.875 2.25 2.5 2.75
None Slow Rapid Rapid
X
xx
xx
-2.2 -1.9 -1.3 -0.9
-
DISCUSSION OF RESULTS The results of the experiment presented are in general agreement with previous publications on the isoelectric precipitates of aluminum and/or iron phosphate, silicate (6),humate, and with soil complexes (3, 7). Each complex would exhibit a more or less definite isoelectric pH, depending upon the proportion of trivalent cation in the newly formed complex. Interfering electrolytes were a t a minimum, and only one exchangeable cation (sodium) was present in the soils of the experiments described. It was previously shown (3) that the divalent exchangeable cations lower the negative charge of soil colloids; thus a smaller amount of coagulant was necessary for the flocculation of calcium soil than for sodium soil, whereas identical quantities of coagulant were required for isoelectric precipitation. Since the composition of soils varies, it is obvious that the composition of stream turbidities will vary from place to place; consequently different sets of conditions will be required for good coagulation. Factors such as the effect of the quantity and type of electrolytes in turbid waters were not considered. Bartow ( I ) , Black (a), and Peterson (IO) found that sulfates as neutral salts present during the precipitation of iron and aluminum hydroxides shift the pH zone of rapid flocculation to the acid side. Theriault and Clark ( l a ) , Miller (9), and others found similar effects with other anions. This was particularly true of di- and trivalent anions in combination with monovalent cations, which is in agreement with the previous work of one of the authors (6, 7) on the isoelectric pH values of various iron and aluminum silicate,. phosphate, and humate complexes. The behavior of a soil complex to coagulant
FIGURE4. RELATION BETWEEN PH AND ELECTROPHORETIC MIGRATION VELOCITYOR COAGULANT (STOCKTON COLLOID COMPLEX)
drolytic acidity is produced, resulting in a lower pH. Furthermore, the more completely hydrolyzed the trivalent cation, the less is its positivity or flocculating power. It would appear, therefore, that the factors of low pH adjustment and mixing are far more important for coagulation with iron salts than with aluminum salts. CONCLUSION
Using a concentration of soil colloids equivalent to 400 p. p. m., it was found that the higher the percentage of iron and aluminum oxides in soils the less coagulant (aluminum chloride) was required to render them isoelectric at a given pH. Each soil colloid can be isoelectrically precipitated over a wide range of hydrogen-ion concentration. With a minimum of coagulant the aluminum-soil complex will be isoelectric a t a pH of 4.0, and any additional quantities of coagulant will raise the isoelectric p H ultimately to that of pure alumina. The treatment of a soil colloid with equivalent quantities of ferric chloride and of aluminum chloride show that the soil colloid treated with ferric chloride retained a stronger negative charge and the resultant pH was lower than the soil colloid treated with aluminum chloride. Flocculation was not as rapid or as complete a t the pH employed with ferric chloride as with aluminum chloride when small quantities were compared.
January. 1935
I N I) I i S 'I' R I A I,
A N 1)
E N Ci
I,ITEHATUS~E CITED i.l .l Bartow, E., Blaak, A,, and Sanabury, W. IW. Ex". (:am.. 25.901 (1933). (2) Black, A., Itiee, 0.. and Bartow, E.. ibid., 25. 811 (lYd.%: (3) Mattson, S..J . Am. Soc. Ayron., 18,458(1026). (4) \Iattson,S.,J.Phz,s. Chrn.,32, 1532 (1928). (5) Msttaon, S.. Soil Sci.. 25,289 (1028). ,.,
~
( 8 ) IlA.,32.343(1931). (9) Miller. L. B., Pub. Hcaith Ileyls., 40,361 (1925). (10) Petorson, B., and Hartow, E.,IND,ENQ.Caax.. 20,5l (1I)W.
Bleeding of Cements lielationship between Bleeding Tendency of Various ;\Jornial arid Poly, Purpose Cements and Water Gain under the Aggregate in Concretes Made from Them LEVI S. RHOWN, Massachusetts Institute of Technology, Cambridge, Mass. HE tendericy of water to rise to the top of concrete soon
T
after it has been placed, here spoken of as "bleeding," is well recognized. Also, various investigators have noted that the bond between the cement paste of a concrete and the aggregate in the concrete, as a rule, is much tighter over the upper surface of the individual pieces of aggregate. Those who have studied these phenomena regard them both as effectsof gravitational settlement or compaction wbile the concrete mass is still fluid. Undoubtedly such compaction tends to express or squeeze out excess water, and it is apparent that in concrete some of the water, in rising toward the top surface of the mass, is entrapped under the bottom surface of the pieces of aggregate in such a way that a lateral flow around the pieces of aggregate is necessary for continued unward mieration. .. tfnder such condit i o n s a f i l m of water tcnds to lie along the hottom surface of the piece
I
: 1. S ~ i n n s nPauu~aFOR
PASTE
The examinatiori qf sezieral cornni,ereiulnormal and TDA Poly-Purpose cements by "bleeding" tests, and the petrographical cxaminatiorr of the hardened concrete showed that the separation of uater from poured concrete and the presence of chanrlels un&r the aggregate in the hardened concrete are different manifestations of the same phmonienon. Bleeding and water gain under the aggregate, while controlled lo some degree by finer grinding or by a reduelion of the water-cement ratio, are reduced to negligible amounts by use of cenienls ground in the presence of a dispersing r\, aynu,, n1 ,UN of aggregate. This fihii uf water is spoken of as "water gain under the aggregate." As the concrete sets and hardens, this water is slowly reabsorbed during the later stages of the
MIXING Concrete pillails reach to bed ruck.
