ADSORPTION OF DYES BY SOILS J. A. WILKINSON AND WILBUR HOFF
The method of determining the amounts of colloidal matter in clays and soils by measuring the amount of some dye taken up has been developed by Rohland’, Ashley2,Lord3 and others. The method is based on the assumption that the dye is taken up by adsorption which is a property oi surfaces and hence is a measure of the colloidal matter. Therefore, the amount of dye taken up is considered a measure of the colloidal matter in the soil. Bancroft4has shown that the dyeing of textiles is essentially an adsorption phenomenon and the dyeing of soils should be of a similar nature. The amount of dye taken up will depend on the following factors; (a) nature of the soil, (b) nature of the dye, (c) acidity or alkalinity of the soil solution, (d) acidity or alkalinity of the dye solution, (e) presence of other salts in solution, (f) a possibility of base exchange between the dye and the minerals in the soil. (a) & (b) From Bancroft’s theory that dyeing is an adsorption phenomenon, a soil containing a positive colloid such as ferric hydroxide will adsorb more of an acid dye such as diamine sky blue than it will if it contains a negative colloid such as silicic acid. This is what is found experimentally by Beaumont5. Similarly a basic dye such as methylene blue will be more adsorbed by a negative colloid like silicic acid. Rohland6 has shown that the amount of dye adsorbed is in general greater the more complex its nature and the more colloidal it is. The dyes usually used for determining the amounts of colloids in clays and soils have been basic dyes such as malachite green chloride or oxalatc, methyl violet, methylene blue, crystal violet and aniline blue. These wit1 measure chiefly the negative colloids. It is obvious therefore that the nature of both the dye and the soil colloid will have a large effect on the amount of adsorption. (c) As Bancroft has shown with textiles an acid dye will be more adsorbed from an acid solution because of the high adsorption of the H ion by the fiber and a basic dye will be less adsorbed from an acid solution than it will be from a neutral or alkaline solution due to the high adsorption of OH ions from the latter. Rohland’ has shown that the amount of methyl orange (an acid dye) taken up by a given soil is greater from an acid solution than from one which is neutral or alkaline. He concludes that the acid forms more colloidal material in the soil but the more probable explanation is that the dye anion is more highly adsorbed due to the H ion of the acid being first adsorbed. Rohland: Farben-Zeitung 18, 5 2 2 (1913). Trans. Am. Cer. SOC.11, 530 (1909). 3Lord: U. S. Bur. of Public Roads. Laboratory Tests for determining the Physical Properties of Subgrade Soils. 4Bancroft: J. Phys. Chem. 18, I , 118,385 (1914). Beaumont: Cornell ilg. Exp. Sta. Mem. 21, 494 (1919). e Rohland: Kolloid-Z. 16, 16 (1915). iRohland: Glasind. 26, Xos. 19-22 (1915).
* Ashley:
ADSORPTIOS O F DYES BY SOILS
809
(d) The amount of dye taken up will depend not only on the acidity or alkalinity of the soil solution when it js treated with water but also on the acidity or alkalinity of the dye solution used. If one had a soil that gave a basic solution on treatment with water the amount of an acid dye taken up from this solution would probably be very small. (e) Just as H ions cause a variation in the amount of dye adsorbed other ions, especially metals, will have a similar effect and a soil or clay that will give a water solution containing salts of Ca, Ilg, Fe, and A1 will adsorb more of an acid dye than will one that gives none of these. Similarly, the presence of anions as nitrate, sulphate or chloride in the soil solution will increase the amount of a basic dye that will be adsorbed'. I n the case of malachite green which is sold either as the oxalate or the chloride the amount of dye adsorbed will be different depending upon which one is used. It should be less in the case of the chloride since the bivalent negative oxalate is more adsorbed than the univalent chloride and hence will cause more of the dye ion to be adsorbed. (f) A basic dye such as malachite green has in addition to the adsorption of the positive dye ion a possibility of exchanging some of its positive ion for the metal ions of the soil particle and thus fix more dye by chemical action or base exchange. Ashley2says that this is the way malachite green is taken up rather than by adsorption. With these points in view the experiments were planned to find the effect of the concentration of acid and base upon the amount of dye adsorbed by different soils and to look for evidence of any base exchange between the dye and the soil. It was thought that by adding varying amounts of acid to the soil one should be able to saturate the soil colloids with H ions and thus increase the amount of an acid dye held to a maximum and decrease the amount of a basic dye held and by using a base increase the amount of a basic dye held and decrease the amount of an acid dye. Six soils were used, ( I ) and ( 2 ) were Ransanian clays, (3) and (4) were Iowa drifts, (5) a black loam and (6) a, dark red sandy clay very high in iron content. An endeavor was made to find a dye that had the same color in acid and alkaline solution but none was found. Xext search mas made for dyes that had definite colors in both acid and alkaline solutions even though the colors were different. The two that were used were diamine blue 3 B and methylene blue. The former is blue in acid solution and purple in neutral and alkaline solution. The latter is blue in neutral and acid solution and pink in alkaline solution. Keutral violet, which is violet in neutral and acid solution but i s insoluble in an alkaline solution, was used also for neutral and acid solutiorl. Diamine blue 3 €3 is an acid dye while the other two are basic dyes. 1
Bancroft: loc. cit. p. 120 Ashley: Bur. Standards Tech. Paper, 23, 41 (1914).
J. A. WILKINSON AND WILBUR HOFF
810
Procedure The amount of dye adsorbed was determined by placing one gram of the soil in a large test tube and adding sufficient dye solution to saturate it and still leave some excess which was determined colorimetrically using a Duboscq colorimeter. The test tubes were then stoppered with paraffined corks and placed in a shaking machine and shaken vigorously for a given time. The soil was then allowed to settle, the concentration of the dye solution determined and the loss taken as the dye adsorbed. To determine the time necessary to
reach equilibrium a series mas shaken for different lengths of time and the adsorption measured. This was done with several dyes and different soils.wLThe following table is typical of the results.
TABLE I Rate of Adsorption of Dye by Soil Time (hours) 0. j
gm dye adsorbed
o
0.0453 0,0457
I
0 .0475
2
0.0482 0.0483 0.0487 0.0489
0.2 j
3 6
8
gm dye adsorbed
0.0455 0.0457 0.0475 0.0483 0.0484 0.0488 0.0489
811
ADSORPTJON O F DYES BY SOILS
TABLE I1 Adsorption of Diamine Blue 3 B from Acid Solution gm of dye adsorbed per gram of soil (!e. I N HCI
Soil ( I )
Soil
0
O.OOI~
O.OOI~
0.0008
0.0045
(2)
Soil (3)
Soil (4)
Soil
Kone Sone 0.0029
(j)
Soil (6)
I
o.oo~j
0.0050
2
0.0034
0.0080
O.OOj0
Kone 0.0029 0.0039
5
0.0050
0.0125
0.0100
o.ooj9
0.0108
IO
0.006;
0.0158
0.0080
0.0115
0.0134 0.0134
0.0100
2.5
0.0143 0,0134
n'one 0.0029 0.0029 0.0029 0.0029
0.0100
0.0050
50
0.0090
0.0100
0.0125
0.0125
0.0091
0.0050
TABLE I11 Adsorption of Diamine Blue 3 B from Alkaline Solution Diamine Blue 3 B adsorbed per gm of soil cco.IN KCH Soil ( I ) Soil (2) Soil (3) Soil (4) Soil (5) 0 0.0007 Sone o.oorj None Xone I
0.001g
2
None
5
0.0007
IO
1'
0.001g
50
0.0025
0.001g
"
0.001g
11
0.001 j
25
0.0015
14
0.0030
11
0.0040
0.0029
0.0040
Soil (6)
Sone
"
'i
11
11
"
'1
11
1'
'L
0.0029 0.0039 0.0039
"
''
ii
'L
11
1'
TABLE ITT Adsorption of Neutral Violel; gm of dye adsorbed per gram of soil cco I
N HCI
Soil
0
0.0791
0.0728
I
0.0758
0.0723
2
0.0692 0.0653 0.0630 0.0603 0.0603
0.0657 0.0600 0.0650
5 IO
25
50
(I)
Soil
(2)
o.o_;so
0.0600
Soil (5)
Soil (6)
0.0605
0.0113
0.0750
0.0531
0.0113
0.0710
0.0527
0.0113
0.0713
0.0713
0.0100
0.0758 0.0758 0.0806
0.0727
0.0485 0.0490
0.07jo
O.O~IO
0.0781
0,0531
0.0137 0.0142
Soil (3)
0.0806 0.0781 0.0727
Soil (4) 0.0750
0.0123
TABLE V Adsorption of Methylene Blue gm of dye per gram of soil x0.1
N HCl
Soil
0
0.0425
I
0.0479
2
0.0479 0.0477 0.0476 0.0467 0.0462
5 IO 25
50
(I)
Soil
(2)
0.0430 0.0437 0.0413 0.0413 0.0413 0.0407 0.0372
Soil (3)
Soil (4)
Soil (5)
Soil (6)
0.0575 0.0586 0.0600 0.0575 0.0584 0.0600
0.0568
0.0360 0.0384 0.0366 0.0294 0,0313 0.0313 0.0360
0.0073 0.0073 0.0073 0.0073
0.0600
0.0568 0.0518 O.Oj20
0.0557
0.0586 0.0586
0.0084 0.0097 0.0104
812
J . A . WILKINSON AND WILBUR HOFF
In all cases it was found that over 95y0of the total dye adsorbed was taken up in an hour of shaking and in later experiments an arbitrary time of one hour was used in determining the amount of dye adsorbed. This agrees with the results of Morse and Curry’ who found that while most of the dye was taken up in an hour or so it did not reach an equilibrium until after 7 2 hours. The Effect of Acidity and Alkalinity The effect of acidity and alkalinity was determined by mixing varying quantities of 0.1N HCl or Na0I-I with water and dye solution so that the total volume in each case was 50 cc and adding to this one gram of soil. The amount of dye that will be taken up is of course dependent upon the excess of dye pres-
FIG.2 Adsorption of Neutral Violet
ent and so a preliminary run was made with each soil to find the approximate amount of dye that would be taken up and slightly more than this amount was introduced for the final run so that the adsorption was always determined in the presence of a very small excess of the dye. After shaking vigorously for an hour in a mechanical shaker the samples were allowed to settle and the concentration of the dye remaining in the liquid determined. The results obtained are shown in Tables 11, 111, IV and T’, and in Figs. I , 2 , 3 and 4. Discussion of Results Diamine Blue 3B is an acid dye, being the sodium salt Xa4C34H24N6014S4, and should therefore be more adsorbed from an acid solution and it is found to be so with every soil that was examined as is shown in Table I1 and Fig. I . The form of the curves agrees with what one should get since an increase in the H concentration will increase the amount of dye adsorbed up to a certain point New Hampshire Agr. Expt. Sta. Reports 19 and 2 0 , p.
275
(1908).
ADSORPTION O F DYES BY SOILS
813
and beyond that the adsorption of the negative C1 will tend to replace the negative dye ion and the amount of dye held will be less. It will be observed that the soils arrange themselves in the same order for all the dyes, the only exception being Soil KO. I with diamine blue in acid solutions. The red sandy clay shows the least adsorption in every case and the Iowa drift soils the most. Table I11 shows that the addition of base decreases the amount of diamine blue taken up compared to that from the acid solution. With two of the soils no dye was adsorbed at all and with one it is only when the solution is O.IK with respect to the base that adsorption took place. With the other
I
-0
I
0
FIG.3 Adsorption of Methylene Blue
three soils the amount is only about one third as much as from acid solutions. This adsorption may be due to the presence of a positive colloid in the soil or to the sodium ion of the base being adsorbed enough to partly overcome the effect of the OH ion. Table I V and Fig. 2 for neutral violet, a basic dye of the formula C14Hl,N,HC1show curves that are the reverse of those for diamine blue 3B. A small amount of acid decreases the dye adsorption very appreciably except with the red sandy clay No. 6, which has but little colloidal matter present as shown by the small amount of adsorption with any of the dyes. After the decrease every soil increases again due to the adsorption of C1 ion partly overcoming the effect of the H ion. Table V and Fig. 3 for methylene blue, which is also a basic dye of the formula Cl,H1,N3S C1, show the same type of curves as did neutral violet although the effect is not so pronounced, possibly because the amount of dye adsorbed is less. In Fig. 4 the values are plotted for four dye solutions with one of the soils (No. I ) . In addition to the data already given in the tables a curve is shown
814
J. A , WILKINSON AND WILBUR HOFF
for another dye, methylene violet, in an alkaline solution. This is a basic dye and should therefore be more adsorbed from a basic solution and this is what it is found to be. Also as the concentration of the base increases the Na ion apparently has a retarding effect as it should.
Base Exchange When a paste was made with each one of the soils using distilled water the solution was found to be slightly alkaline to litmus. It was noticed that if an amount of methylene blue or neutral violet was added, which was insufficient to saturate the clay but left the solution still colorless, it was more alkaline
- 1
CC.0./ N AC/D OR 2 A S f ADD.0 FIG.4 Adsorption of Dyes by Soil No.
I
than it had been with pure water. A qualitative examination of these solutions showed the presence of calcium, magnesium and potassium. Each soil was then tested to see if they showed any base exchange between the colored ion of the dye and the metal ions of the soil. Two grams of the clay were treated with 50 cc of conductivity water and shaken for a period of several days and then allowed to stand until the supernatent liquid was clear. This in some cases required several weeks. Another sample was treated with insufficient amount of dye to quite saturate it so that the liquid remaining behind was clear. This was shaken in the same way and allowed to stand. After pipetting off the solutions they were analyzed for Ca, Mg and K and in every case more of all these were found in the solutions where the dye was used than in the water solutions. A quantitative determination was made with Soil No. I and methylene blue for the amount of Ca and R/lg replaced. With water alone z grams of the soil
ADSORPTION O F DYES BY SOILS
81;
gave 0.0346 grams of Ca while with the dye it gave 0.0570 grams. The amount of Mg found was but a trace, being 0.0002 grams, but none at all was found in the pure water solution. A further quantitative study is being made of the effect of base exchange.
Conclusions ( I ) The adsorption of dyes by soils and clays is of the same nature as the dyeing of fibers. ( 2 ) The amount of dye taken up may be increased or decreased by varying the acidity or alkalidity of the solution. (3) Xnety-five per cent of the total dye adsorbed will be taken up during an hour of steady shaking. (4) There is some evidence of base exchange between the dyes and the basic elements in the soil. Chemical Laboratory Iowa State College Ames, Iowa.
