ADSORPTION BY METALLIC HYDROXIDES. I ADSORPTIOS O F ARSENIOUS ACID BY PRECIPITATED FERRIC HYDROXIDE BY KSHITlSK CHANDRA SEN
General Introduction The study of adsorption of various substances by means of hydrated oxides as adsorbents has been done mainly from two points of view, namely ( I ) to compare their adsorptive power with adsorbents such as animal charcoal, etc, and ( 2 ) whether any relation between the coagulating power and adsorption of ions exists or not. In the first case, mainly the adsorption of dyes and in some cases that of acids and salts has been examined whereas in the second case both salts and acids were used with a colloidal solution of the hydroxide. As an instance of the first case, a paper by Freundlich and Poser,' in which a comparative study of the adsorptive power of alumina, bole and animal charcoal on some basic and acid dyes, and of salts of the alkaloids and certain other organic bases was made, may be mentioned. The papers by Gann,2Ishizaka,s and those by Weiser and his collaborator^>^ are examples of the second type. There is anot,her investigation which has drawn the attention of several investigators, namely the adsorpt,ion of arsenious acid by hydrated ferric oxide. Besides these-though stray cases of adsorption by hydrated iron, aluminium and chromium oxides appear in the literature, it seems that no systematic attempt has been made to make a comparative study of the adsorptive power of these hydroxides for inorganic and organic acids. In a previous paper,5 I have shown that when a ferric chloride solution is mixed with sufficient quantity of arsenious acid, and then caustic alkali is added, no precipitation of the ferric hydroxide occurs, but instead a colloidal solution of negatively charged ferric hydroxide is obtained. In a later paper,6 the same behaviour with chromium hydroxide was also shown. These results indicated that the stabilisation of the hydroxides may be due to ths adsorption of the negatively charged arsenite ions, and hence about two years ago an investigation on the adsorptive power of these hydroxides for arsenious acid was started. The results obtained mere of sufficient interest t o extend the investigation t o other acids, mainly organic, t,he experiments being done with all the three oxides under varying conditions. The object of this series of papers is to present the results obtained, and their possible applicat,ion t o the theoretical problems of adsorption and colloid coagulation has been pointed out. Kolloidchem. Beihefte, 6, 297-328 (19141. Rolloidchem. Eeihefte, 8, 63 (1916). Z. physik Chem., 83, 97 (1913). J. Phys. Chem., 24, 30, 630 (1920). J. Phys. Chem., 28, 313 (1924). Iiolloid-Z., 34, 262 (1924).
420
KSHITISH CHAKDRA BEN
Adsorption of Arsenious Acid by Ferric Hydroxide The adsorption of arsenious acid by ferric hydroxide has been studied by several authors. The investigation was started by Bunsen and Berthold,’ who observed the great adsorption power of ferric hydroxide for arsenious acid and considered the combination to he a basic ferric arsenite. Seventy years after the publication of this result, the subject was quantitatively investigated by Biltz2 who concluded that it was a case of adsorption and not chemical combination, since it follows the so-called adsorption law expressed by a equation of the type (x/m)” = kC, where x i m is the amount of the solute taken up per gram of the adsorbent, C is the concentration in the solution a t equilibrium and n and li are constants. In his experiments Biltz found that the values of n and k were respectively j and 0.631. In lat,er years, Lockemann and Paucke,s and Lockemann and Luciud studied the conditions under which the adsorption of arsenious acid from solution is complete. The results obtained bv the latter authors show that a given amount of alsenious acid is removed from solution by a considerably smaller amount’ of precipitate when an exactly stoichiometric amount of ammonia is used than when twice that amount is used. They also observed that the adsorption of arsenious acid is considerably reduced by the use of potassium hydroxide or sodium hydroxide instead of ammonia. Mecklenburgj worked with ferric hydroxide and arsenious acid in demonstrating his principle of “affine hdsorptionskurven” i. e. adsorption curves, so related, each by a particular factor, to an unit curve, that the ordinate corresponding to any given abscissa on one of the curves may he obtained by multiplying, by the factor, the ordinate on the unit curve corresponding to the given abscissa. He showed that the adsorption curves obtained from differently prepared samples of hydrated ferric oxide were actually related in such a way to an unit curve, which he at first determined for one particular preparation. In a recent paper Bosnell and Dickson6 have again studied the subject and have come to several interesting conclusions. They used a two-month old sample of precipitated ferric hydroxide and they did not find any ageing effect of the hydroxide on its adsorption power. The adsorption of arsenious acid was found to be diminished by the presence of sodium hydroxide, but the adsorption of sodium hydroxide was found to be increased by the presence of arsenious acid. In supporting further the generalisation of Mecklenburg to the effect that where the adsorption of a compound from solution follows the equation of the type E = PAP, p is a constant for the same adsorbing material, and that an adsorbent prepared under various conditions, or of various ages will vary only in t’he value of /3, Boswell and Dickson have shown that p remains constant for different preparations of ferric hydroxide where arsenious oxide alone was used. In Table I the results calculated for various preparations are shown. “Das Eisenhvdroxvd,” (18.31). Ber., 37, 313h, (1964). Kolloid-Z., 8 , 273 (191I j Z. physik. Chem., 83, 735 (1913). 6 2 . phvsik. Chem., 83. 6.9 (19131. ti J. :am. Chem. SOC.,40, 1793 (1918). 1
ADSORPTIOS BY METALLIC HYDROXIDES
42 I
TABLE I Constants for the equation E = Series
Boswell and Dickson i r ) pure ;Is?OS ( 2 ) AispOs-constant SaOH 1Iecklenburg I O Do 11°-120 Do 23 Do 45O Do 51' Do 60'
Do
paP for ferric hydroxide P
Precipitation temperature 0.210
128
0.284
333 195 184
0.183 0.188 0.197
0.186 0 .19.5 0.187
184 I 80 170
IjO
0.19.;
I20
Do unit curve
98O -
0 .I88
2 00
Do air-dried Biltz
-
98O
0.19j 0.198
43 Ij0
These results support the conclusions of Llecklenburg t o a certain extent. As the results obtained by me are somewhat different, an account now will be given of the effect of ageing, different method of preparation, etc.. on the adsorptive power of ferric hydroxide under different conditions.
Ezperimental melhori. The ferric hydroxide was always precipitated a t the room temperature (about 2 5°C). from a 2 S solution of FeClr with excess of dilute ammonia, care being taken t o avoid any rise in the temperature by immersing t'he whole vessel in cold water. The precipitate was usually highly gelatinous, and it was washed part,ly by decantation and finally hy dialysis till completely free from chloride. The freshly precipitated substance was dark brown, but when one sample was allowed t o "age" under water: after four months it became dull red. .AI1 the samples \yere kept always covered with water. I n making the adsorption experiments, I O cc of this hydroxide was taken out by means of a pipette, and introduced into a I O O cc flask. The necEsssry amount of t,hs solute was then poured into the flask, and water was added up to the mark. The whole thing was shaken several times a t constant intervals and then allowed t o settle for a fixed time. The supernatant liquid c as then analysed iodometrically in presence of sodium hi-carbonate. LIany observations showed that there was no appreciable error in taking out the ferric hydroxide by means of the pipette, the maximum being 0.2 per cent. I n course of the investigation, it was found advisable t o use a constant quantity of some neutral salt like KC1 in the flasks, otherwise a peptisation of the hydroxide took place, and the supernatant liquid became turbid. I t was however found out that the presence of this salt exerted no effect on the amount of adsorption by hydrated ferric oxide. In some preliminary cases, ammonium chloride has been used as the salt. Where acids or alkali have been used as the
KSHITISH CHAXDRA SEX
422
solute, they were titrated by means of carbonate free alkali or standard acid with phenolphthalein as the indicator. The time of settling was usually 2 0 hours and the volume loo cc. From preliminary experiments it was found that the adsorption reqults were quite reproducible. I n Table I1 the effect of ammonium chlorideisshown. TABLEI1 Concentration of .A.s?03 in grms. 0.
S o SH4C1
Adsorption in grams. sIIIcl=.o13 mole litre
0.09228
I j78
2624 0,09468 0.06312
mole litre. 0.092~8
0.08748
0.08748
0 .I
SH,CI= .OZ
0.07448
0.07448 0.06236
0.06236
0.07448
0.06236
These results show that the addition of ammonium chloride has no effect on the final equilibrium and potassium chloride also behaves similarly. At this place, some experiments were made t o test the reversibility of the equilibrium between arsenious acid and ferric hydroxide. Biltz (loc. cit.) has already shown that the equilibrium is reversible and my experiments have also supported it. It will he shown later on that the effect of time on this adsorption is also nil when three hours are allowed for the adsorbent t o settle down. I n the following table the effect of concentration of the arsenious acid on its adsorption by a fixed amount of ferric hydroxide is shown. The hydroxide is the same as used in the previous experiments. TABLE 111 Adsorption of arsenious acid wt. of Fez03 = 0.2645 grms. Crigiilal concentration .4s203in grms
0.4901
0,4593 0.39i7 0.336i 0,2840 0.2jz48
0.2209 0.18936
Concentration after adsorption
Adsorption in grms hy I gr adsorbent
0.4688
0,3661 0.3363 0.2793
0.44i7
0.2223
0.4325
0.1967
0.40ji
0.14968 0.1193
0.3890 0.3841
0 . I jj8
0.0909 0.08j5 2
0.12624
0.038;6
0,09468 0.0631 2
0.02020
0.00076
0.46jo
0.3i23
0,3469 0.3308 0.2816 0 . 2 3 j6
The hydroxide used in the above investigation was prepared only a few days before the actual experiment was made. Its colour rvas dark brown, and this sample was now put inside a stoppered bottle and kept in a dark place for four months in order t o investigate the effect of ageing. The temperature
ADSORPTIOF BY METALLIC HYDROXIDES
423
of the outer atmosphere rose about 6' higher during this interval. On opening the vessel after about four months, it was found that the colour of the sample was now dull red, and though the hydroside was always kept under water, it appeared to have been dehydrated and agglomerated. Though this is curious, it seems to be the only explanation of this remarkable colour change.' The weight of ten cubic centimeters of the substance as withdrawn by the pipette remained constant, and in Table IT' are the results obtained by using this sample as the adsorbent of arsenious acid is given. Other conditions remain the same :
FIQ.I
TABLE 11' Original conc. of .lszO, in grms.
Conc. after adsorption
0.3638 0.34178 0,31947 0,29739 . o . 2 7 508 0.2369
0.30584 0.28865 0.26680 0.24610
0.20861
Adsorption in gms. by I gr. of the adsorbent
0.2192 0.2008 0.I992 0 . I939
0.22770
0 . I792
0.19021
0.176j
0.16215
0.1757
The results given in Tables I11 and IV are plotted in Fig. I . It will be obvious that the ageing has an appreciable effect on the adsorptive power of ferric hydroside and t,hat the actual amount of adsorption greatly decreases due to this ageing effect. It has already been mentioned that, Boswell and Dickson did not find any ageing effect on their ferric hydroxide. My results are therefore different from theirs and it became a matter of interest to investigate this subject further. It, has been found that t>hisageing effect is quite appreciable after the first few days of the preparation of the oxide. hut Keiser: J. Phys. Chem., 24, 286
(1920).
424
KSHITISH C H A S D A A SEX
after about 2 : days, t,he effect of ageing is not apprecialile. This means t h a t the hydroxide changes at first rather quickly. hut after sometinie this change is very slowv. In the light of these experiments, the result of Boswell and Dickson becomes apparent, for these investigators used a two-month old hydroxide for adsorption esperinients. I n Taiile T>the adsorptive power of the fresh hytlrositle is conipared to that of the aged hydroxide when the original concentration of the arsenious acid is the same. The values have been obtained from the esperiniental curves.
TABLE 1Original ctinc. or hs20ain grin..
Fresh hydroxide
Amount adsorbed tiy one gram of the aged hydroside 0.096; 0 .I80
0 .IO
0.300
0.15
0.2.;
0.347 0.3;3 0.3Yi
0.30
0.41;
0.197
0.40
0.44;
0.280
0.90
0 .
rho
0 . 180
It will thiis be oliserved that the amount of adsorption 1)y an old saiiiple of ferric hytlroxide is aliout fifty per cent less than t h a t of the fresh one. Efecf oj lime. I n all the resultr; given aiio\-e. the time :illon.ctl for the cqiiililiriuiii t o set in has been 2 0 hours. - i t this point it became tiesirablc t o investigate the effect of time, if any, on the equilihriuin concentrations. I f m y esperinients were made which showed that if the hydroxitie particles :ire allon-et1 to stttt,lc completely, the final concentration of the solute in thti ,mpcrnatant liquitl is always the sanne. I n Talile 1.1 is the result olitainetl n.ith the aged sninple.
TABLE TI .\mount a d s o r h d per gram adsorbent
Original conc. of AsrOsiii prni. 0.2369
Time 3 hours 0.1765
Time
20
hours
0.176;
Tlik rwult shon R that the cquili1)rium was reached within three hours. and the atlsorption does not change with the increase in the duration of the contact of thc solute with the aclsorlient.
E r p e r i m x f s with t l i f c r r u t snmples In the experimental results already recorded, the same material under two different conditions has been esamined. It was now thought desirable t o investigate whether 1IeclclenIiurg's generalisation as to the constancy of the value of the exponential factor in the adsorption equation is true for our sarnples or not. F o r this experiiiient, two other samples of ferric hydroside were prep a i d . the niethotl of preparation lieing almost the same as before. E'or the sake of convenience the hytlroxitle already exanlined will he tlenotetl as saiiiple So. I , whilrt the agctl one g ill he denoted a? So. la. In l'al)le 1-11 the resnlts ohtainetl with t h e sample S o . 2 are shoirn.
42 5
ADSORPTIOS BY METBLLIC HYDROXIDES T A B L E TI1 Fez03= 0.4719 gr. S H I C l = . 0 2 mole per litre Vol. = I O O cc. Time 2 0 hours.
Or,iginal conc. in grms. 0.
ji13
o.jz30
0.6;48 0.4820 0
4338
0
3856 33i4
0
Conc. of the solution after adsorption 0.6365 0,5896 0.4420
Amount adsorbed by I grm. adsorbent. 0.2856 0,2828 0 . 2
0.3; .2 2 o 30j1 o 2621 o 2176
j96
0.2751 0.2j28
0.261j 0 . 2A:.
I
I n Table T.111, the results iyith sample S o . 3 is shown. In this esperiment, ainiiioninm chloride was replaced by potassinni chloride. the concentration of which was 0.0; mole per litre. Pot,assiiim chloride 1ion.ei-er had no effect on the equilibrium between the hytlrositie anti the arscnious acid. The amount of Fe.03 used u-ne 0.2539 gr. T h e other conditions remain the mine. TAI~L 1.111 E (Original conc. of .-\s103 grms. o 2892 2410
0
Conc. of the solution after adsorption 2270
0.2131
0 .I 8 0 2
0 . 2 3 76
0
1928 0 . I 446 0
@,0(,84
Amount adsorbed I grm. sdsorhent
tiy
0 2322 0.2224 0 .
04.;6
0,
1986
In l’alile I S t h e results olitainctl 1)y varying t,he amount of adsorlient S o . 3 have lieen recortictl. Tlic original concentration of the arsenious acid 0 . 2 8 9 2 gr. v-as the sanie in all C I ~ S C B . the ninoiint of ferric hydroside actually taken tieing in each case different. It i- intcresting to note the change in tlie actual ainount of adsorption calculated per gram of the atlsorbent n-ith the change in the aiiioiint of the ndsorlmit taken for esperinier This result is shown in Fig. 2 as S o . 3a. T.%ULE I S ;\mount of adsorbent in grms.
(’onc of t h r solution s f t r r adsorption
__
0.2892
__
0,2.;.
