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Sorption of metals on humic acid
(IOlh-7017 80
I lO-l7olso?i)o
0
Sorption of metals on humic acid* H. KERNDORFF~ and M. SCHNITZER$ Chemistry
and Biology
Research Institute, Agriculture Ontario, Canada KIA OC6
Canada,
Ottawa,
Abstract--The sorption on humic acid (HA) of metals from an aqueous solution containing Hg(II). Fe(III). Ph. Cu, Al. Ni, Cr(III), Cd, Zn. Co and Mn, was investigated with special emphasis on effects of pH. metal concentration and HA concentration. The sorption efficiency tended to increase with rise in pH. decrease in metal concentration and increase in HA concentration of the equilibrating solution At pH 2.4. the order of sorption was:
At pH 3.7. the order
Hg > Fe > Pb > Cu=AI iv;,?:
z Ni > Cr=Zn=Cd=Co=Mn
Hg=Fc>AI>Pb>Cu>Cr>Cd=Zn=Nl>Co=Mn, while at pH 4.7 it was: Hg=Fe=Pb=Cu=Al==Cr At pH 5.8. the order
> Cd > Ni=Zn
> Co > Mn.
was: Hg=Fe===Pb=Al=Cr=Cu
> Cd > Zn > Ni > Co > Mn.
Hg and Fe were always most readily removed, while Co and Mn were sorbed least readily. There were indications of competition for active sites (CO,H and nhenolic OH rrrouns) on the HA between the different metals. We were unable to find correlaiions between the affini%es bf’the eleven metals to sorb on HA and their atomic weights, atomic numbers. valencies, and crystal and hydrated ionic radii. The sorption of the eleven metals on the HA could be described by the equation Y = lOOi[l + exp - (A + BX)], where Y = ‘,, metal removed by HA; X = mg HA; and A and B are empirical constants
INTRODUCTION HUMIC XJASTANCES are widely distributed over the earth’s surface. occurring in terrestrial as well as in aquatic environments. One of the most striking characteristics of these materials is their relatively high content of oxygen-containing functional groups (CO,H. phenolic and alcoholic OH, ketonic and quinonoid C = 0) through which they can interact with inorganic soil and water constituents. Humic acid (HA) is that humic fraction that is soluble in dilute base but is coagulated by acidification of the alkaline extract. In aqueous systems, HA is normally insoluble at pH < 6.5. The ability of HA to sorb metals has been demonstrated in a number of investigations (SZALAY. 1964: BASU et al., 1964; CH~WDHURY and BOSE, 1970: RASHID and LEONARD, 1973; RASHID, 1974: BUNZL it al., 1976). Yet much remains to be learned about the nature of these interactions. The purpose of this investigation was to obtain more detailed information on the sorption of metals on HA, with special emphasis on effects of pH, metal concentration. and HA concentration. We examined the sorption on HA of the following metals:
* CBRI No. 1139. f Visiting scientist from the Institute of Geological Sciences. the University of Mainz, Mainz, West Germany. $ Author to whom correspondence should be addressed,
Hg(II), Fe(III), Pb, Cu, Al, Ni, Cr(III), Cd, Zn, Co and Mn. The eleven metals, in dilute aqueous solution, were allowed to interact with the HA simultaneousl) rather than one metal at a time. This procedure was thought to resemble more closely what happens in natural soils and waters and also under conditions of environmental pollution. The results obtained were interpreted in the light of current knowledge on metal-HA interactions. MATERIALS AND METHODS
The HA was extracted from the surface horizon (O-15 cm) of the Bainsville clay loam, a Humic Gleysol. on the Central Experimental Farm at Ottawa. Ontario. After sampling, the soil was air-dried and ground to pass through a 0.25 mm sieve. The methods used for the extraction of the HA were the same as those described by CHEN &~‘t al. (1978). Briefly. the soil was treated with 0.5 N NaOH under N2 for 24 hr at room temperature. The alkaline extract was acidified with 2 N HCI to pH 2 and allowed to stand for 24 hr at room temperature. The coagulate (HA) was separated from the supernatant (fulvic acid = FA) by centrifugation. The ash content of the HA was lowered by the following sequence of procedures: (a) the HA was redissolved in 0.5 N NaOH under N,; the resulting solution was centrtfuged for 20 min at 12,OOOrpm to remove insoluble materials (mainly clay minerals). The clear supernatant was then acidified with 2 N HCI to pH 2 in order to coagulate the HA; (b) the
1701
I702 Table
H. KEKNDORFFand M. SCHNITZLR 1. Analytical
characteristics of the HA ash-free basis)
',C
SH ‘tN “5 %I meq total
meq CO:H/g
acidity/g
meq phenolic
OH/g
ion
a
dry.
53.5 6.2 5.5 0.8 34.0 6.4 3.4 3.0
partly-purified HA was shaken at room temperature for 12 hr with 100 ml of dilute HF--HCl (5 ml of 52?h HF + 5 ml of cont. HCI + 990 ml of distilled H,O) solution. The excess acid xas removed by centrifugation and washing with distilled H,O. This procedure was repeated three times in succession: (c) finally. the HA was transferred to dialysis bags and dialyzed for 216 hr against distilled H,O changed at frequent intervals. Following these treatments. the HA was freeze-dried. The freeze dried HA contained -I’),, moisture and 0.974;, ash. The latter consisted mainly of Na, followed by smaller amounts of Cu. Fe and Ca. Table I sl~ows;I number of analytical characteristics of the HA.
A stock
solution
which was 5 x 10.’ M in each of the from nitrates of Zn. Cd. Hg(II), Cu. FetIII). Co. Ni. Mn. Al. Cr(III) and Pb. The stock solution was diluted with distilled water for the preparation of more dilute solutions.
I1 metals uas prepared
Required amounts of dry HA were dissolved in 3 ml of 0.5 N NaOH in SO ml beakers and diluted to 30 mi with distilled HzO. The solutions were acidified with 0.5 N HNO, to pH 2 in order to coagulate the HA. The fresh coagulates were transferred to 100 ml volumetric flasks to which were added aliauots of metal stock solutions that were 0.5 x 1W3. I.0 X’ 10m4 and 0.5 x IO-“ M in each of the eleven metals. Each solution was diluted with distilled water to 95 ml. the pH adjusted when required with dilute NaOH solution and the volume made to lOOmI with distilled water. Preliminary experiments showed that sorption was essentially complete after shaking at 25 + I C for 2 hr. Thus. a 2 hr equilibration time was adopted. Following this. each suspension was centrifuged for 30 min at 4000 rpm to separate insoluble residues from the supernatams. To aliquots of the clear supernatants. 5 ml of cont. HNO, was added in order to maintain the metals in solution and further dilutions were made with distilled water. Amounts of each metal sorbed were considered to be equai to the differences between quantities of metals added and quantities of metals recovered. Each sorption experiment uas done in triplicate. The coefficient of variation between replicates was 1.9”;,. Points in Figs. 1 to 4 are mean values.
All metals were determined on clear supernatants by atomic absorption spectroscopy. Zn, Cd, Cu, Ni, Co. Fe, Mn and Pb were determined with the aid of an air-acetylene flame; A) in a nitrous oxide-acetylene flame and in a solution containing 2000 ppm of La. For determinations of Zn, Cd and Pb, absorbing and non-absorbing lines as well as a hydrogen continuum lamp were used. Hg was measured by nameless atomic absorption spectroscopy as described by MALAIYANDI and BARETTE (1970). Coefficients of variation between replicate metal analyses averaged I’>,;,
f and H were determined by dry-combustion. N by the automated Dumas method, S by oxygen-flask combustion and 0 was calculated by difference. Total acidity Was measured by allowing the HA to react with an excess of Ba(OH), and back-titrating the unreacted Ba(OH), after completion of the reaction. while carboxyl groups were measured bv the calcium acetate method IS~CINITZER and GLIPTA, 196$ Phenoiic hydroxyis were taken as the difference between total acidity and CO,H groups. Moisture was determined by heating samples at IO.5 C for 24 hr and ash bv ignition at 75O’C lor 4 hr. pH Measurements were made with the aid of a Fisher microprobe combination electrode. RESULTS
AND
DISCUSSlON
The freshly coagulated HA was equilibrated with solutions that contained 0.5 x 10-4mol (Fig. la), 1.0 x IO-’ moi (Fig. fb). and 0.5 x 1O~‘mol (Fig. Ic) of each of the eleven met&. The pH was kept constant at 2.4, while the HA concentration ranged from 0 to IO00 mg. The volume of each system was 100 ml. Fig. la illustrates the sorption behavior of the eleven metals from the most concentrated metal solution. The metals can be subdivided into the following three groups: (a) those that were very strongly sorbed by the HA (Fe and Hg); (b) those whose sorption affinity was intermediate (Cu. Pb and Al); and (c) metals that sorbed weakly on HA under the experimental conditions employed (Ni. Cr, Zn. Mn and Co). As the metal concentration was lowered (Fig. I b), strong sorption of Pb and Cu in addition to Hg and Fe coutd be observed. There was a shght reduction in the removal of Al but increased sorption of the remaining metals in spite of a decrease in the HA weight from 1000 to 500 mg. A further reduction of the metal concentration (Fig. Ic) did not bring about any signihcant changes in the sorption behavior of the metals. The curve for Al was omitted from Fig. Ic becnuse concentrations of Al in the supernatants were too low for reliable measurements,
Effects on sorption of raising the pH from 2.4 to 5.8. while maintaining a constant metal concentration (0.5 x 10V4 mol of each metal in 100 ml of solution) are illustrated in Figs la (pH 2.4). Id (pH 3.7), le (pH 4.7). and If (pH 5.8). As the pH of the system rose, greater proportions of added metals were adsorbed by the HA. At pH 4.7 (Fig. le), 250 mg of HA sorbed practically all the added Fe. Hg, Pb, Cr, Al and Cu. while between 10 and 407; of the Mn. Co, Zn, and Ni was also adsorbed. With further increase m pH to 5.8 (Fig. If), proportions of the latter elements sorbed increased substantially. The sorption data at the four different pH levels are summarized in Table 2. The Table also shows data for the formation of insoluble metal hydroxides at pH 5.8, which were determined in the absence of HA. At the lower pH levels (2.44.7) we did not observe the for-
Sorption of metals on humic acid mation of any insoluble hydroxides after allowing the
solutions to stand for 24 hr at room temperature. From the data in the last vertical column in Table 2 we can attempt to estimate the contributions of insoluble metal hydroxides to sorption. Thus, at pH 5.8, in the cases of Fe. Cu, Cr and to a lesser extent of Al. Hg and Pb, removal of portions of these metals from
1703
the aqueous systems as hydroxides is possible. To find out whether the concentrations of metals in our sorption experiments were of the same order as those found in polluted sediments, we calculated amounts of metals sorbed on 1 g of HA from the data in Table 2 and compared these values with data published in the literature. For example, at pH 4.7. 1 p of HA
Fe
“g
mg HA Fig. 1. Effects of metal concentration of the equilibrating solution and pH on the sorption of metals on HA (a) 0.5 x 10m4mol of each metal; (b) 1.0 x 10-5mol of each metal; (c) 0.5 x 10. 5 mol of each metal; (a) pH 2.4; (d) pH 3.7; (e) pH 4.7; (f) pH 5.8.
H.
1704
KERNDORFF
and M.
SCHNITZER
-*- ..__._ -
-0, l.
\,___-o )..
Hg
Fe
Fig. 2. Effect of HA concentration
Pb
Cu
Al
Ni
Cr
Cd
Zn
CO
.
Mn
on sorption of metals on HA at pH 2.4. Each system contained 0.5 x 10e4 mol of each metal.
PH Fig. 3. Proportions
of metals sorbed by 250 mg of HA at different pH levels. Initial metal content was 0.5 x low4 mol of each metal in 100 ml of solution.
Sorption
of metals on humic acid
. .._
0.
Hg
o
cont.
Fig. 4. Proportions
of metals
x lo-’
moles
sorbed
at pH 3.7 from more concentrated
sorbed 1200 pg of Cu, 360 pg of Ni, and 392 fig of Zn. Concentrations of the same metals in 1 g of fine sediment from the Altrhein river near Mainz, West Germany (LASKOWSKIet d., 1976) ranged as follows: Cu:
pH 3.7
metal solutions.
617776 pg; Ni: 37-728 pg and Zn: 2267,841 pg. Thus, the metal concentrations in our experiments were of magnitudes that were similar to those found in polluted sediments.
Table 2. Effect of pH on sorption pH 2.4
Hg,Fe
‘/‘r
\
metal
1705
pH 4.7
of metals on HA* pH 5.8
pH 5.8
HA Cm91 Metal
1000
500
200
500
200
250
250
100 100 97 95 100 29 100 41 29 22 13
98 100 98 97 100 61 100 77
0
% sorption
Hg Fe Pb cu Al Ni ccdr Zn co Mn
80 88 65 66 59
88 87 33 31 29
;o" 8
3 4'
10 4 5
5 2 2
* Each system contained ** Insoluble hydroxide.
99 81 19 12 7 5 0
98 96 96 91 98 41 E
0 0 0
33 23 14
0.5 x lo-“
98 96 80 :z 7: 7 8 2 3
mol of each metal.
:z 36
24** 88** 24** 52** 38** 0 53** 0 0 0 2**
1706
H. KERNDORFF and M. SCHNITZER
From the data presented in the previous section, it appears that the sorption of metals on HA is affected by the metal concentration, pH of the equilibrating solution and the amount of HA that is present. To obtain a better understanding of the sorption processes. we replotted some of our experimental data. Fig. 3 shows the effect of pH on sorption in a system (100 ml) that contained 250 mg of HA and 0.5 x 1O--4 mol of each metal. The plots indicate that in general the sorption efficiency increased with increasing the pH from 2.4 to 5.8. At pH 4.7, and often more so at pH 5.8, practically all Hg. Fe, Pb, Cu. Al and Cr was removed from the equilibrating solution. Similarly, about 75”/;; of the Cd, 60% of the Zn and Ni but only about 40% of the Co and Mn sorbed on the HA at pH 5.8. Analysis of all sorption data by the Duncan multiple range test (p = 0.05) showed that at pH 2.4, the order of sorption was:
the sum of COzH + phenolic OH groups. was 6.4 m-equivig. This value approximates the concentration of active sites on the HA surface. The HA used in this investigation contained 5.5”/,N (see Table I). About one half of the latter occurs in proteins, the remaining one half as ammonia. amino sugars. nucleic acid bases and possibly in heterocyclic rings. While some free NH,-groups and cyclic N forms may bind metals, most workers consider nitrogen-~ontajning groups in HA to be less active in this respect than functional groups containing oxygen.
There arc indications in a number of figures of competition between different metals for active sites on the HA. The data in Fig. 4 provide such information. In this experiment, 1.0 g samples of HA were equilibrated at pH 3.7 with solutions containing relametal concentrations of each high tively (5-75 x 10-s mol). As the concentr~~tion of each tig > Fe > Pb > Cu=AI z Ni > Cr=Zn==Cd=Co=Mn. surpassed equilibrium solution metal in the At pH 3.7. the order was: 25 x IO- ’ mol. only Hg. Fe, Cr and. to a lesser extent Al. Pb and Cu. remained sorbed on the HA. No Cd, Hg===Fe> Al > Pb > Cu > Cr > Cd=Zn=Ni > Co=Mn, Ni. Zn, Co and Mn was sorbed by the HA at this while at pH 4.7, it was: stage, apparently displaced from the HA by Hg. Fe, Cr, Al, Pb and Cu. As the equilibrating solution Hg===Fe=Pb=Cu=Al=Cr > Cd > Ni=Zn > Co> Mn. became more concentrated. Hg. Fe and Cr displaced At pII 5.8. the order was: Al, Pb and Cu. There are indications in Fig. 4 that at very high metal ~oncentr~~tion. Fe displaced Cr. Of all k(p--=Fe=Pb=Al=Cr=Cu > Cd > Zn r Ni > Co > Mn. elements investigated, Hg appeared to he retained Thus, regardless of pH, Hg and Fe were always most most firmly by the HA and could not be displaced readily sorbed by HA, while Co and Mn were sorbed from it by any of the other 10 metals, least readily. Finally, to throw additional light on the competiThe orders of sorption listed above were similar to tion between metal, we examined the sorption of Cu that reported by BUNZL e? al. (1976) for the selective by HA at pH 2.4 from 2 systems: one containing only uptake of metal ions by hydrogen-saturated peat in Cu, while the other one contained CLI plus the 10 the pH range 3.545, namely, Pb> Cu > Cd=Zn > Ca other metals. The results showed that there was conand are in line with observations of RASHID (1974) siderable competition between Cu and the other that sedimentary and peat HA’s preferentially adsorb metals for active sites on the HA. This was especially Cu over Co, Mn, Ni and Zn. The sorption orders also so when the systems contained small amounts of HA agree with the findings of CHOWDH~RY and BOSE and the number of active sites was limited. (1970) who reported that at pH values prevailing under natural conditions, Pb and Cu were more Mechanism of sorption strongly retained by soil humus than were Zn, Ni and It is apparent from the data presented herein that CO. the system under investigation is a very complex one. Eleven different ions plus protons, that is. 12 ions compete for sorption on the HA. Not only do the 12 One of the most characteristic features of humic ions interact with the HA, but they also mteract with substances is their relatively high content of oxygeneach other. We attempted to develop a treatment ~ollt~iiiling fun~tion~~l groups (especially CO,H and based on thermodynamics to describe these reactions. phenolic OH groups) (SCHNITZEK, 1978). It is through but were unable to do so because of the complexity of the functional groups that these materials are thought the system. It is for this reason that we are reporting to interact with metal ions, metal oxides, metal hyour results in a less rigorous manner. droxides and minerals to form metal-organic associAttempts to correlate the affinity of the Ii metals to ations of widely differing chemical and biological stasorb on the HA surface with their atomic weights, bilities and characteristics, As shown in Table 1, the atomic numbers, valencies, crystal and hydrated ionic HA that was used in this investigation contained per radii, were unsuccessful. CHOWDHURYand BOSE(1970) g 3.4 m-eyuiv of C02H and 3.0 m-equiv of phenolic and RASHID (1974) have suggested that the preferenOH groups. SO that the total acidity, which is equal to tial sorption on HA metals such as Pb and Cu was
loo/(
LOGISTICS
Sorption
of metals on humic acid
I +
-_(A+
EXP
I707
BX))
A=-2
B
2
I””
JUV
3””
Fig. 5. Curves representing the sorption of metals on HA. A is constant. equalling -2
related to the ability of these metals to form stable complexes, possibly chelates, with HA. More recently, different views have been expressed. GAMBLE et al. (1976) have shown by NMR measurements that Mn(II) forms an outer sphere complex with fulvic acid. while Fe(III) forms an inner sphere complex with the same material. Similarly, GAMBLE et al. (1977) and MCBRIDE (1978) concluded from ESR measurements that Mn(I1). rather than forming chelates with humic materials, was bound in fully hydrated form [as Mn(H,O)i’] by electrostatic bonding only. An outer sphere complex is one in which the ion is bound electrostatically to the polyanion (such as HA) without displacement of water of coordination from the ion by a negatively charged functional group. In an inner sphere complex, by contrast, ligand functional groups may enter into coordination positions and displace strongly coordinated H,O molecules. MCBRIDE (1978) concluded from ESR investigations that HA does not form highly covalent metalorganic bonds with Cu nor with Mn. He believes that a single electrostatic bond formed between the HA and metals is consistent with the ESR results, which appears to put some doubt on chelation as a predominant mechanism. Regardless of which mechanism is the more relevant one, our data show significant differences in the sorption of the 11 metals on HA. depending on the pH, metal and HA concentration. Modrls,f&r the sorptim
qf metals on HA
By curve fitting a family of curves was drawn from
the experimental data to model the sorption of metals on HA (Fig. 5). The equation for these curves, calculated on the computer, is: Y = lOO,![l + exp - (A + SX)]
(11
where: Y = % metal sorbed by HA X=mgHA A and B = empirical constants Effects of varying constants B and A arc shown in Fig. 5. If A and B are high, the slopes are steep and the curves are close to the ordinate (curves V. U. T. S and R). But if A and B are low, the curve is less steep and is far from the ordinate (curve N). Medium values for A and B yield curves Q. P and 0. Curves V to R in Fig. 5 are representative of the sorption on HA of Fe. Hg. Cr and AI (See Fig. le), curves Q to 0 of the sorption of Pb and Cu and curve N of the sorption of Cd, Ni. Zn, Co and Mn. If the pH of the system increases, the slopes increase because of rapid increases of B. while A changes little or decreases. If the metal concentration increases. B tends to remain almost constant while A tends to decrease slightly, so that the slope of the curves changes little.
.4~knorv(ed~r,tfrrlts- -We thank B. ZAUWK for technlcal assistance, K. PKI(I.L of the Engineering and Statistical Research institute for statistical analyses and the German Academic Exchange Service (DAAD) for financial assistance to H.K.
H. KEREDORFF and M. SCHNIPZW
1708 REFERENCES
BASI: A. N., MUKHERJEE D. C. and MUKHERJEE S. K. (1964) Interaction between humic acid fraction of soil and trace element cations. J. Iudiun Sot. Soil Sci. 12, 31 I-318. BU&;ZL K.. SCHMIDT W. and SANSONI B. (1976) Kinetics of ion exchange in soil organic matter IV. Adsorption and desorption of Pb“, Cu’+. Cd”. Zn*’ and Ca’+ by peat. J. Soil SC;. 27. 32~ 41. CHEN Y., SE.N~SI N. and S~HYXITZERM. (1978) Chemical and physical characteristics of humic and fulvlc acids extracted from soils of the Mediterranean region. Grodc>rnlu 20, 87FlO4. CHOWDHLIRY A. N. and BOSE B. B. (1970) Role of ‘humus matter’ in the formation of geochemical anomalies. Proc. 3rd IN. Geochm~. Erplor. Synp.. Toronto, pp. 41&413. GAMBLE D. S.. LAUGFORI) C. H. and TONG J. P. K. (1976) The structure and equilibria of a manganese (11) complex of fulvic acid studied by ion exchange and nuclear magnetlc resonance. CU,I. J. Cllem. 54, 1239~1245. GAMHLE D. S.. SCHNITZER M. and SKINNER D. S. (1977) Mn(II)-fulvic acid complexing equilibrium measurements by electron spin resonance spectrometry. C’rr~t.J. Soil SC;. 57. 47 53. LASI(OWSKI N.. KOST T. H.. POMMERFNKE K.. SWAFEH A.
and TOBS~HALL H. I. (1976) Abundance and distribution of some heavy metals in recent sediments of a highly polluted limnlc-fluviatile ecosystem near Mainz, West Germany. In Ellrironmrnral Biogeochc~n~r.srr~. (ed. J. 0. Nriagu). Vol. 2, pp. 5X7- 595. Ann Arbor. MALAIYASIX M. and BARET~L J. (1970) Determination of submicro-quantities of mercury m biological materials. A,1trl. Lelr. 3, 579 5x4. M~BRII~I M. B. (1978) Transition metal bondmg in humic acid. An ESR study. Soil Sci. 126. ?0&209. RASHII) M. A. (1974) Absorption of metals on sedimentary and peat humic acids. Chrrn Geol. 13, I 15- 123. R4SHID M. A. and LtO~uAKIIJ. D. (1973) Modifications of the solubllity and precipitation behaviour of various metals as a result of their interactions with sedimentar) humic acid. Cheln. Geoi. 11. X9-97. STHNITZF.R M. (1978) Humic substances: Chemistry and reactions. In Soil Orymic Mtrrrw. (eds M. Schnitzer and S. U. Khan), pp. I 64. Elsevier. SCHXTLER M. and GUPTA U. C. (1965) Determination of acidity in soil organic matter. SOI/ SC,;. Sot. Am. Proc,. 29, 274~ 277. SZALAV A. (1964) Cation exchange propertlcs of humic acids and their importance in the geochemical enrichment of UOS _ and other cations. G~&~ir,l. Cosmochim Actu 28, 160.5 1614.
I lO-l7olso?i)o
0
Sorption of metals on humic acid* H. KERNDORFF~ and M. SCHNITZER$ Chemistry
and Biology
Research Institute, Agriculture Ontario, Canada KIA OC6
Canada,
Ottawa,
Abstract--The sorption on humic acid (HA) of metals from an aqueous solution containing Hg(II). Fe(III). Ph. Cu, Al. Ni, Cr(III), Cd, Zn. Co and Mn, was investigated with special emphasis on effects of pH. metal concentration and HA concentration. The sorption efficiency tended to increase with rise in pH. decrease in metal concentration and increase in HA concentration of the equilibrating solution At pH 2.4. the order of sorption was:
At pH 3.7. the order
Hg > Fe > Pb > Cu=AI iv;,?:
z Ni > Cr=Zn=Cd=Co=Mn
Hg=Fc>AI>Pb>Cu>Cr>Cd=Zn=Nl>Co=Mn, while at pH 4.7 it was: Hg=Fe=Pb=Cu=Al==Cr At pH 5.8. the order
> Cd > Ni=Zn
> Co > Mn.
was: Hg=Fe===Pb=Al=Cr=Cu
> Cd > Zn > Ni > Co > Mn.
Hg and Fe were always most readily removed, while Co and Mn were sorbed least readily. There were indications of competition for active sites (CO,H and nhenolic OH rrrouns) on the HA between the different metals. We were unable to find correlaiions between the affini%es bf’the eleven metals to sorb on HA and their atomic weights, atomic numbers. valencies, and crystal and hydrated ionic radii. The sorption of the eleven metals on the HA could be described by the equation Y = lOOi[l + exp - (A + BX)], where Y = ‘,, metal removed by HA; X = mg HA; and A and B are empirical constants
INTRODUCTION HUMIC XJASTANCES are widely distributed over the earth’s surface. occurring in terrestrial as well as in aquatic environments. One of the most striking characteristics of these materials is their relatively high content of oxygen-containing functional groups (CO,H. phenolic and alcoholic OH, ketonic and quinonoid C = 0) through which they can interact with inorganic soil and water constituents. Humic acid (HA) is that humic fraction that is soluble in dilute base but is coagulated by acidification of the alkaline extract. In aqueous systems, HA is normally insoluble at pH < 6.5. The ability of HA to sorb metals has been demonstrated in a number of investigations (SZALAY. 1964: BASU et al., 1964; CH~WDHURY and BOSE, 1970: RASHID and LEONARD, 1973; RASHID, 1974: BUNZL it al., 1976). Yet much remains to be learned about the nature of these interactions. The purpose of this investigation was to obtain more detailed information on the sorption of metals on HA, with special emphasis on effects of pH, metal concentration. and HA concentration. We examined the sorption on HA of the following metals:
* CBRI No. 1139. f Visiting scientist from the Institute of Geological Sciences. the University of Mainz, Mainz, West Germany. $ Author to whom correspondence should be addressed,
Hg(II), Fe(III), Pb, Cu, Al, Ni, Cr(III), Cd, Zn, Co and Mn. The eleven metals, in dilute aqueous solution, were allowed to interact with the HA simultaneousl) rather than one metal at a time. This procedure was thought to resemble more closely what happens in natural soils and waters and also under conditions of environmental pollution. The results obtained were interpreted in the light of current knowledge on metal-HA interactions. MATERIALS AND METHODS
The HA was extracted from the surface horizon (O-15 cm) of the Bainsville clay loam, a Humic Gleysol. on the Central Experimental Farm at Ottawa. Ontario. After sampling, the soil was air-dried and ground to pass through a 0.25 mm sieve. The methods used for the extraction of the HA were the same as those described by CHEN &~‘t al. (1978). Briefly. the soil was treated with 0.5 N NaOH under N2 for 24 hr at room temperature. The alkaline extract was acidified with 2 N HCI to pH 2 and allowed to stand for 24 hr at room temperature. The coagulate (HA) was separated from the supernatant (fulvic acid = FA) by centrifugation. The ash content of the HA was lowered by the following sequence of procedures: (a) the HA was redissolved in 0.5 N NaOH under N,; the resulting solution was centrtfuged for 20 min at 12,OOOrpm to remove insoluble materials (mainly clay minerals). The clear supernatant was then acidified with 2 N HCI to pH 2 in order to coagulate the HA; (b) the
1701
I702 Table
H. KEKNDORFFand M. SCHNITZLR 1. Analytical
characteristics of the HA ash-free basis)
',C
SH ‘tN “5 %I meq total
meq CO:H/g
acidity/g
meq phenolic
OH/g
ion
a
dry.
53.5 6.2 5.5 0.8 34.0 6.4 3.4 3.0
partly-purified HA was shaken at room temperature for 12 hr with 100 ml of dilute HF--HCl (5 ml of 52?h HF + 5 ml of cont. HCI + 990 ml of distilled H,O) solution. The excess acid xas removed by centrifugation and washing with distilled H,O. This procedure was repeated three times in succession: (c) finally. the HA was transferred to dialysis bags and dialyzed for 216 hr against distilled H,O changed at frequent intervals. Following these treatments. the HA was freeze-dried. The freeze dried HA contained -I’),, moisture and 0.974;, ash. The latter consisted mainly of Na, followed by smaller amounts of Cu. Fe and Ca. Table I sl~ows;I number of analytical characteristics of the HA.
A stock
solution
which was 5 x 10.’ M in each of the from nitrates of Zn. Cd. Hg(II), Cu. FetIII). Co. Ni. Mn. Al. Cr(III) and Pb. The stock solution was diluted with distilled water for the preparation of more dilute solutions.
I1 metals uas prepared
Required amounts of dry HA were dissolved in 3 ml of 0.5 N NaOH in SO ml beakers and diluted to 30 mi with distilled HzO. The solutions were acidified with 0.5 N HNO, to pH 2 in order to coagulate the HA. The fresh coagulates were transferred to 100 ml volumetric flasks to which were added aliauots of metal stock solutions that were 0.5 x 1W3. I.0 X’ 10m4 and 0.5 x IO-“ M in each of the eleven metals. Each solution was diluted with distilled water to 95 ml. the pH adjusted when required with dilute NaOH solution and the volume made to lOOmI with distilled water. Preliminary experiments showed that sorption was essentially complete after shaking at 25 + I C for 2 hr. Thus. a 2 hr equilibration time was adopted. Following this. each suspension was centrifuged for 30 min at 4000 rpm to separate insoluble residues from the supernatams. To aliquots of the clear supernatants. 5 ml of cont. HNO, was added in order to maintain the metals in solution and further dilutions were made with distilled water. Amounts of each metal sorbed were considered to be equai to the differences between quantities of metals added and quantities of metals recovered. Each sorption experiment uas done in triplicate. The coefficient of variation between replicates was 1.9”;,. Points in Figs. 1 to 4 are mean values.
All metals were determined on clear supernatants by atomic absorption spectroscopy. Zn, Cd, Cu, Ni, Co. Fe, Mn and Pb were determined with the aid of an air-acetylene flame; A) in a nitrous oxide-acetylene flame and in a solution containing 2000 ppm of La. For determinations of Zn, Cd and Pb, absorbing and non-absorbing lines as well as a hydrogen continuum lamp were used. Hg was measured by nameless atomic absorption spectroscopy as described by MALAIYANDI and BARETTE (1970). Coefficients of variation between replicate metal analyses averaged I’>,;,
f and H were determined by dry-combustion. N by the automated Dumas method, S by oxygen-flask combustion and 0 was calculated by difference. Total acidity Was measured by allowing the HA to react with an excess of Ba(OH), and back-titrating the unreacted Ba(OH), after completion of the reaction. while carboxyl groups were measured bv the calcium acetate method IS~CINITZER and GLIPTA, 196$ Phenoiic hydroxyis were taken as the difference between total acidity and CO,H groups. Moisture was determined by heating samples at IO.5 C for 24 hr and ash bv ignition at 75O’C lor 4 hr. pH Measurements were made with the aid of a Fisher microprobe combination electrode. RESULTS
AND
DISCUSSlON
The freshly coagulated HA was equilibrated with solutions that contained 0.5 x 10-4mol (Fig. la), 1.0 x IO-’ moi (Fig. fb). and 0.5 x 1O~‘mol (Fig. Ic) of each of the eleven met&. The pH was kept constant at 2.4, while the HA concentration ranged from 0 to IO00 mg. The volume of each system was 100 ml. Fig. la illustrates the sorption behavior of the eleven metals from the most concentrated metal solution. The metals can be subdivided into the following three groups: (a) those that were very strongly sorbed by the HA (Fe and Hg); (b) those whose sorption affinity was intermediate (Cu. Pb and Al); and (c) metals that sorbed weakly on HA under the experimental conditions employed (Ni. Cr, Zn. Mn and Co). As the metal concentration was lowered (Fig. I b), strong sorption of Pb and Cu in addition to Hg and Fe coutd be observed. There was a shght reduction in the removal of Al but increased sorption of the remaining metals in spite of a decrease in the HA weight from 1000 to 500 mg. A further reduction of the metal concentration (Fig. Ic) did not bring about any signihcant changes in the sorption behavior of the metals. The curve for Al was omitted from Fig. Ic becnuse concentrations of Al in the supernatants were too low for reliable measurements,
Effects on sorption of raising the pH from 2.4 to 5.8. while maintaining a constant metal concentration (0.5 x 10V4 mol of each metal in 100 ml of solution) are illustrated in Figs la (pH 2.4). Id (pH 3.7), le (pH 4.7). and If (pH 5.8). As the pH of the system rose, greater proportions of added metals were adsorbed by the HA. At pH 4.7 (Fig. le), 250 mg of HA sorbed practically all the added Fe. Hg, Pb, Cr, Al and Cu. while between 10 and 407; of the Mn. Co, Zn, and Ni was also adsorbed. With further increase m pH to 5.8 (Fig. If), proportions of the latter elements sorbed increased substantially. The sorption data at the four different pH levels are summarized in Table 2. The Table also shows data for the formation of insoluble metal hydroxides at pH 5.8, which were determined in the absence of HA. At the lower pH levels (2.44.7) we did not observe the for-
Sorption of metals on humic acid mation of any insoluble hydroxides after allowing the
solutions to stand for 24 hr at room temperature. From the data in the last vertical column in Table 2 we can attempt to estimate the contributions of insoluble metal hydroxides to sorption. Thus, at pH 5.8, in the cases of Fe. Cu, Cr and to a lesser extent of Al. Hg and Pb, removal of portions of these metals from
1703
the aqueous systems as hydroxides is possible. To find out whether the concentrations of metals in our sorption experiments were of the same order as those found in polluted sediments, we calculated amounts of metals sorbed on 1 g of HA from the data in Table 2 and compared these values with data published in the literature. For example, at pH 4.7. 1 p of HA
Fe
“g
mg HA Fig. 1. Effects of metal concentration of the equilibrating solution and pH on the sorption of metals on HA (a) 0.5 x 10m4mol of each metal; (b) 1.0 x 10-5mol of each metal; (c) 0.5 x 10. 5 mol of each metal; (a) pH 2.4; (d) pH 3.7; (e) pH 4.7; (f) pH 5.8.
H.
1704
KERNDORFF
and M.
SCHNITZER
-*- ..__._ -
-0, l.
\,___-o )..
Hg
Fe
Fig. 2. Effect of HA concentration
Pb
Cu
Al
Ni
Cr
Cd
Zn
CO
.
Mn
on sorption of metals on HA at pH 2.4. Each system contained 0.5 x 10e4 mol of each metal.
PH Fig. 3. Proportions
of metals sorbed by 250 mg of HA at different pH levels. Initial metal content was 0.5 x low4 mol of each metal in 100 ml of solution.
Sorption
of metals on humic acid
. .._
0.
Hg
o
cont.
Fig. 4. Proportions
of metals
x lo-’
moles
sorbed
at pH 3.7 from more concentrated
sorbed 1200 pg of Cu, 360 pg of Ni, and 392 fig of Zn. Concentrations of the same metals in 1 g of fine sediment from the Altrhein river near Mainz, West Germany (LASKOWSKIet d., 1976) ranged as follows: Cu:
pH 3.7
metal solutions.
617776 pg; Ni: 37-728 pg and Zn: 2267,841 pg. Thus, the metal concentrations in our experiments were of magnitudes that were similar to those found in polluted sediments.
Table 2. Effect of pH on sorption pH 2.4
Hg,Fe
‘/‘r
\
metal
1705
pH 4.7
of metals on HA* pH 5.8
pH 5.8
HA Cm91 Metal
1000
500
200
500
200
250
250
100 100 97 95 100 29 100 41 29 22 13
98 100 98 97 100 61 100 77
0
% sorption
Hg Fe Pb cu Al Ni ccdr Zn co Mn
80 88 65 66 59
88 87 33 31 29
;o" 8
3 4'
10 4 5
5 2 2
* Each system contained ** Insoluble hydroxide.
99 81 19 12 7 5 0
98 96 96 91 98 41 E
0 0 0
33 23 14
0.5 x lo-“
98 96 80 :z 7: 7 8 2 3
mol of each metal.
:z 36
24** 88** 24** 52** 38** 0 53** 0 0 0 2**
1706
H. KERNDORFF and M. SCHNITZER
From the data presented in the previous section, it appears that the sorption of metals on HA is affected by the metal concentration, pH of the equilibrating solution and the amount of HA that is present. To obtain a better understanding of the sorption processes. we replotted some of our experimental data. Fig. 3 shows the effect of pH on sorption in a system (100 ml) that contained 250 mg of HA and 0.5 x 1O--4 mol of each metal. The plots indicate that in general the sorption efficiency increased with increasing the pH from 2.4 to 5.8. At pH 4.7, and often more so at pH 5.8, practically all Hg. Fe, Pb, Cu. Al and Cr was removed from the equilibrating solution. Similarly, about 75”/;; of the Cd, 60% of the Zn and Ni but only about 40% of the Co and Mn sorbed on the HA at pH 5.8. Analysis of all sorption data by the Duncan multiple range test (p = 0.05) showed that at pH 2.4, the order of sorption was:
the sum of COzH + phenolic OH groups. was 6.4 m-equivig. This value approximates the concentration of active sites on the HA surface. The HA used in this investigation contained 5.5”/,N (see Table I). About one half of the latter occurs in proteins, the remaining one half as ammonia. amino sugars. nucleic acid bases and possibly in heterocyclic rings. While some free NH,-groups and cyclic N forms may bind metals, most workers consider nitrogen-~ontajning groups in HA to be less active in this respect than functional groups containing oxygen.
There arc indications in a number of figures of competition between different metals for active sites on the HA. The data in Fig. 4 provide such information. In this experiment, 1.0 g samples of HA were equilibrated at pH 3.7 with solutions containing relametal concentrations of each high tively (5-75 x 10-s mol). As the concentr~~tion of each tig > Fe > Pb > Cu=AI z Ni > Cr=Zn==Cd=Co=Mn. surpassed equilibrium solution metal in the At pH 3.7. the order was: 25 x IO- ’ mol. only Hg. Fe, Cr and. to a lesser extent Al. Pb and Cu. remained sorbed on the HA. No Cd, Hg===Fe> Al > Pb > Cu > Cr > Cd=Zn=Ni > Co=Mn, Ni. Zn, Co and Mn was sorbed by the HA at this while at pH 4.7, it was: stage, apparently displaced from the HA by Hg. Fe, Cr, Al, Pb and Cu. As the equilibrating solution Hg===Fe=Pb=Cu=Al=Cr > Cd > Ni=Zn > Co> Mn. became more concentrated. Hg. Fe and Cr displaced At pII 5.8. the order was: Al, Pb and Cu. There are indications in Fig. 4 that at very high metal ~oncentr~~tion. Fe displaced Cr. Of all k(p--=Fe=Pb=Al=Cr=Cu > Cd > Zn r Ni > Co > Mn. elements investigated, Hg appeared to he retained Thus, regardless of pH, Hg and Fe were always most most firmly by the HA and could not be displaced readily sorbed by HA, while Co and Mn were sorbed from it by any of the other 10 metals, least readily. Finally, to throw additional light on the competiThe orders of sorption listed above were similar to tion between metal, we examined the sorption of Cu that reported by BUNZL e? al. (1976) for the selective by HA at pH 2.4 from 2 systems: one containing only uptake of metal ions by hydrogen-saturated peat in Cu, while the other one contained CLI plus the 10 the pH range 3.545, namely, Pb> Cu > Cd=Zn > Ca other metals. The results showed that there was conand are in line with observations of RASHID (1974) siderable competition between Cu and the other that sedimentary and peat HA’s preferentially adsorb metals for active sites on the HA. This was especially Cu over Co, Mn, Ni and Zn. The sorption orders also so when the systems contained small amounts of HA agree with the findings of CHOWDH~RY and BOSE and the number of active sites was limited. (1970) who reported that at pH values prevailing under natural conditions, Pb and Cu were more Mechanism of sorption strongly retained by soil humus than were Zn, Ni and It is apparent from the data presented herein that CO. the system under investigation is a very complex one. Eleven different ions plus protons, that is. 12 ions compete for sorption on the HA. Not only do the 12 One of the most characteristic features of humic ions interact with the HA, but they also mteract with substances is their relatively high content of oxygeneach other. We attempted to develop a treatment ~ollt~iiiling fun~tion~~l groups (especially CO,H and based on thermodynamics to describe these reactions. phenolic OH groups) (SCHNITZEK, 1978). It is through but were unable to do so because of the complexity of the functional groups that these materials are thought the system. It is for this reason that we are reporting to interact with metal ions, metal oxides, metal hyour results in a less rigorous manner. droxides and minerals to form metal-organic associAttempts to correlate the affinity of the Ii metals to ations of widely differing chemical and biological stasorb on the HA surface with their atomic weights, bilities and characteristics, As shown in Table 1, the atomic numbers, valencies, crystal and hydrated ionic HA that was used in this investigation contained per radii, were unsuccessful. CHOWDHURYand BOSE(1970) g 3.4 m-eyuiv of C02H and 3.0 m-equiv of phenolic and RASHID (1974) have suggested that the preferenOH groups. SO that the total acidity, which is equal to tial sorption on HA metals such as Pb and Cu was
loo/(
LOGISTICS
Sorption
of metals on humic acid
I +
-_(A+
EXP
I707
BX))
A=-2
B
2
I””
JUV
3””
Fig. 5. Curves representing the sorption of metals on HA. A is constant. equalling -2
related to the ability of these metals to form stable complexes, possibly chelates, with HA. More recently, different views have been expressed. GAMBLE et al. (1976) have shown by NMR measurements that Mn(II) forms an outer sphere complex with fulvic acid. while Fe(III) forms an inner sphere complex with the same material. Similarly, GAMBLE et al. (1977) and MCBRIDE (1978) concluded from ESR measurements that Mn(I1). rather than forming chelates with humic materials, was bound in fully hydrated form [as Mn(H,O)i’] by electrostatic bonding only. An outer sphere complex is one in which the ion is bound electrostatically to the polyanion (such as HA) without displacement of water of coordination from the ion by a negatively charged functional group. In an inner sphere complex, by contrast, ligand functional groups may enter into coordination positions and displace strongly coordinated H,O molecules. MCBRIDE (1978) concluded from ESR investigations that HA does not form highly covalent metalorganic bonds with Cu nor with Mn. He believes that a single electrostatic bond formed between the HA and metals is consistent with the ESR results, which appears to put some doubt on chelation as a predominant mechanism. Regardless of which mechanism is the more relevant one, our data show significant differences in the sorption of the 11 metals on HA. depending on the pH, metal and HA concentration. Modrls,f&r the sorptim
qf metals on HA
By curve fitting a family of curves was drawn from
the experimental data to model the sorption of metals on HA (Fig. 5). The equation for these curves, calculated on the computer, is: Y = lOO,![l + exp - (A + SX)]
(11
where: Y = % metal sorbed by HA X=mgHA A and B = empirical constants Effects of varying constants B and A arc shown in Fig. 5. If A and B are high, the slopes are steep and the curves are close to the ordinate (curves V. U. T. S and R). But if A and B are low, the curve is less steep and is far from the ordinate (curve N). Medium values for A and B yield curves Q. P and 0. Curves V to R in Fig. 5 are representative of the sorption on HA of Fe. Hg. Cr and AI (See Fig. le), curves Q to 0 of the sorption of Pb and Cu and curve N of the sorption of Cd, Ni. Zn, Co and Mn. If the pH of the system increases, the slopes increase because of rapid increases of B. while A changes little or decreases. If the metal concentration increases. B tends to remain almost constant while A tends to decrease slightly, so that the slope of the curves changes little.
.4~knorv(ed~r,tfrrlts- -We thank B. ZAUWK for technlcal assistance, K. PKI(I.L of the Engineering and Statistical Research institute for statistical analyses and the German Academic Exchange Service (DAAD) for financial assistance to H.K.
H. KEREDORFF and M. SCHNIPZW
1708 REFERENCES
BASI: A. N., MUKHERJEE D. C. and MUKHERJEE S. K. (1964) Interaction between humic acid fraction of soil and trace element cations. J. Iudiun Sot. Soil Sci. 12, 31 I-318. BU&;ZL K.. SCHMIDT W. and SANSONI B. (1976) Kinetics of ion exchange in soil organic matter IV. Adsorption and desorption of Pb“, Cu’+. Cd”. Zn*’ and Ca’+ by peat. J. Soil SC;. 27. 32~ 41. CHEN Y., SE.N~SI N. and S~HYXITZERM. (1978) Chemical and physical characteristics of humic and fulvlc acids extracted from soils of the Mediterranean region. Grodc>rnlu 20, 87FlO4. CHOWDHLIRY A. N. and BOSE B. B. (1970) Role of ‘humus matter’ in the formation of geochemical anomalies. Proc. 3rd IN. Geochm~. Erplor. Synp.. Toronto, pp. 41&413. GAMBLE D. S.. LAUGFORI) C. H. and TONG J. P. K. (1976) The structure and equilibria of a manganese (11) complex of fulvic acid studied by ion exchange and nuclear magnetlc resonance. CU,I. J. Cllem. 54, 1239~1245. GAMHLE D. S.. SCHNITZER M. and SKINNER D. S. (1977) Mn(II)-fulvic acid complexing equilibrium measurements by electron spin resonance spectrometry. C’rr~t.J. Soil SC;. 57. 47 53. LASI(OWSKI N.. KOST T. H.. POMMERFNKE K.. SWAFEH A.
and TOBS~HALL H. I. (1976) Abundance and distribution of some heavy metals in recent sediments of a highly polluted limnlc-fluviatile ecosystem near Mainz, West Germany. In Ellrironmrnral Biogeochc~n~r.srr~. (ed. J. 0. Nriagu). Vol. 2, pp. 5X7- 595. Ann Arbor. MALAIYASIX M. and BARET~L J. (1970) Determination of submicro-quantities of mercury m biological materials. A,1trl. Lelr. 3, 579 5x4. M~BRII~I M. B. (1978) Transition metal bondmg in humic acid. An ESR study. Soil Sci. 126. ?0&209. RASHII) M. A. (1974) Absorption of metals on sedimentary and peat humic acids. Chrrn Geol. 13, I 15- 123. R4SHID M. A. and LtO~uAKIIJ. D. (1973) Modifications of the solubllity and precipitation behaviour of various metals as a result of their interactions with sedimentar) humic acid. Cheln. Geoi. 11. X9-97. STHNITZF.R M. (1978) Humic substances: Chemistry and reactions. In Soil Orymic Mtrrrw. (eds M. Schnitzer and S. U. Khan), pp. I 64. Elsevier. SCHXTLER M. and GUPTA U. C. (1965) Determination of acidity in soil organic matter. SOI/ SC,;. Sot. Am. Proc,. 29, 274~ 277. SZALAV A. (1964) Cation exchange propertlcs of humic acids and their importance in the geochemical enrichment of UOS _ and other cations. G~&~ir,l. Cosmochim Actu 28, 160.5 1614.