- Email: [email protected]
Influence of different concentrations of Al2(SO4)3 and anionic polyelectrolytes on tannery wastewater flocculation
DESALINATION Desalination 171 (2004) 13-20
ELSEVIER
www.elsevier.com/locate/desal
Influence of different concentrations of A12(804) 3 and anionic polyelectrolytes on tannery wastewater flocculation v
Zeljko Bajza*, Petra Hitrec, Marko MuSic Faculty of Chemical Engineering and Technology, Marulidev trg 19, 10000 Zagreb, Croatia TeL +385 (1) 483-3850; Fax +385 (1) 466-7526; email: [email protected]
Received 8 September 2003; accepted 1 April 2004
Abstract
Tarming of hides and skins to convert them into leather can have a considerable environmental impact. Wastewater treatment technology can successfully purify leather wastewater. The first part of this research investigated the influence ofAl2(SO4)3concentration on the velocity of tannery wastewater settling. The wastewater sample was taken from two of the most polluted wastewater flows after liming and after chrome tanning. These wastewater flows were mixed together in a ratio of 1:1. The second part was carried out with the samples of wastewater mixed together with different concentrations of Alz(SO4)3. The behaviour of settling was investigated after an addition of different concentrations of anionic polyelectrolytesin these samples. The pH, suspended solids mass, volume of sediment, y(Na2S), ~,(Cr203), chemical oxygen demand, along with turbidity in the supernatant were determined in the obtained sludge. The results demonstrate the influence ofA12(SO4)3and anionic polyelectrolyte concentrations on the parameters studied and the improvement of environmental properties. Keywords: Tanneries; Wastewater; A12(SO4)3;Anionic polyelectrolyte; Liming; Chrome tanning;Sludge properties
1. Introduction
The liming process is the first major step in leather making. The pelt has to be freed of the epidermis and hair, including the hair roots, and the keratinous material filling the hair follicles. The group of proteins known as keratins differs from collagen, elastin and many other hide pro*Corresponding author.
teins in that they have a much greater content of cystine residues forming inter- and intra-protein crosslinks. These S-S bridges can be split by reduction or oxidation quite selectively without influencing the collagen fibre network. Reduction can be performed by almost any kind ofreductive agents, preferably under alkaline conditions. The most widely used compounds are reductive sulphur- or thio- compounds. They act by ex-
0011-9164/04/5- See front matter © 2004 Elsevier B.V. All rights reserved doi: 10.1016/j.desal.2004.04.003
14
Z,. Baiza et al. / Desalination 171 (2004) 13-20
changing with one of the sulphur atoms in the disulphide bridge of cystine [ 1]. O
O
II
If
---C-HN-CH-C-NH--
II
o
---C-HN-CH-C-NH--'
I
IJ
c.
o
S + 2HS -- R
I O
tJ
(1)
~"
When the ionic strength = 0 at 27°C, the value of k is 2.15+0.15x10 -4 min -l. The measured dissociation constant of the hexaaquo-chromic cation is [3] Ka = [Cr(H20)S(OH) 2+ [H30 ] + [Cr(H20)6]
S--S--R
CIH
o
I
HN - C H - C
c.
(4)
S--S--R
S
-'-C-
I
6k [Cr(H20)6] 3+
II II
-Nil ---
O
"-C-IIN
I
CH
= -NH ---
II
Chrome tanning, as it is commonly called in the leather trade, is the most widely applied tanning process world-wide, accounting for between 70% and 80% of all leather production. The trivalent chromium ion, Cr 3+, has outstanding tanning properties, allowing the manufacture of a wide range of leathers with remarkable mechanical, physical and chemical characteristics as well as high shrinkage temperatures. The chromium salt most frequently used for this purpose is the basic hydrated sulphate, which has the formula [2]: (2)
In terms of space, six ligands take positions on the angles of the octahedron. The charge of that complex, in dissociated form, is 3+. Because of warming, water molecules are pressed back and changed with anions from outside the complex [3]. The question, how strong is the bond between water molecules and chromium in water solution?, can be answered by carrying out an exchange reaction between these ions in ~80-enriched water: [Cr(H20)6] 3+ + H2'80 ~ [Cr(H2Os) (H2'80)] 3+ + H20
(5)
J -CtI-C
O
[Cr (H20)5OH]SO 4
10 -3"8z
3+
The kinetics of chrome tanning is complicated. Relative initial reaction rates for typical limed collagen may be estimated from the Handerson equation: pKa = pH +log {[HAft[A-]}
(6)
if it is assumed that the pKa of the sidechain carboxyls is 4 and that reaction only occurs at ionized carboxyls [5]. Traditional leather tanning processes involve treating the hides with an aqueous solution of chromium. Suppose the wastewater from a tannery contains 26 mg/L chromium, originally in the Cr 3+ state. As the effluent flows downstream, the dissolved oxygen can oxidize Cr 3+ to Cr2072-. We calculated the extent of oxidation for a situation where oxygen in the stream water is in equilibrium with atmospheric oxygen and has a pH of 6.5 (a H30* = 10-6"5). The first step is to calculate the concentrations of Cr 3÷ and Cr2072assuming that the chromium species are also in equilibrium with the system [6]. The relevant reaction for atmospheric 02 in equilibrium with water (often called a well aerated system) is: O2(g ) + 4H30 + (aq) + 4e - ,y----'
6H20
E ° = 1.23 V (3) E° -
The reaction rate for one water molecule is defined by constant k.
1.23 V
0.0591 V
p E ° = 14.1
- 20.8
(7)
Z. Bajza et al. /Desalination 171 (2004) 13-20
For the'. Cr system: Cr202- (aq) + 14H30 + (aq) +6e-
,__'
2Cr3+(aq)+17H20
p E ° = 23.0
(8) l log
pE=pE°--6
[Cr3*]2 [Cr2072-]ta +,,4 H30 ]
Since the chromium and oxygen systems are in equilibrium, p E is the same for both: Cr3+]2
=
1.6 × 10 -38
(9)
This very :smallratio indicates that virtually all of the Cr3+would be oxidized to Cr2072- and this, in fact, has serious environmental consequences [6]. Coagulation/flocculation mechanisms: Coagulation/flocculation followed by clarification is the most widely used process for treating wastewater from tannery processing. The process usually consists of the rapid dispersal of a coagulant into the wastewater followed by an intense agitation commonly defined as rapid mixing. The most widely used coagulants are aluminium(III) and iron(III) salts [7]. Liquid aluminium sulphate is a 49 wt% solution of A12( $ 0 4 ) 3 × 14 H20, or about 8.3 wt% aluminium as AI203 [8]. Aluminium salts are effective in removing a broad range of impurities from water, including colloidal particles and dissolved organic substances. Their mode of action is generally explained in terms of two distinct mechanisms: charge neutralization of negatively charged colloids by cationic hydrolysis products and incorporation of impurities in an amorphous hydroxide precipitate. The relative importance of these mechanisms depends on factors such as pH and coagulant dosage. At around neutral pH, AI(III) has limited solubility because of the precipitation of an amor-
15
phous hydroxide, which can play a very important role in practical coagulation and floeculation processes. More importantly in practice, hydroxide precipitation leads to the possibility of sweep floceulation, in which impurity particles become enmeshed in the growing precipitate and thus effectivelyremoved. The aluminium hydroxide is of very low solubility, and an amorphous precipitate can form at intermediate pH values. This is of enormous practical significance in the action of these materials as coagulants. With a further increase in pH, the soluble anionic form Me(OH)4 becomes dominant. As well as the simple monomeric hydrolysis products discussed above, there are many possible polynuclear forms that can be considered. Mononuclear AI species (Alo) react almost instantaneously and polynuclear species (Alb) much more slowly. The varying charge with pH can greatly affect the precipitation process. At around neutral pH for aluminium, the initially formed colloidal precipitate is positively charged and, hence, is colloidally stable. As the pH is increased towards the IEP, the stability decreases and the particles can aggregate into large, settleable floes. The presence of highly charged anions, such as sulfate, can have a large effect on hydroxide precipitation. Sulfate can reduce the positive charge of the precipitate in the acid region so that large floes are formed over a wider pH range. Over the usual range of natural water pH (say, 5-9) particles nearly always carry a negative surface charge. Because of their surface charge, aquatic particles are often colloidally stable and resistant to aggregation. For this reason, coagulants are needed to destabilise the particles. According to the classical ideas of colloid stability, destabilisation can be brought about by either: • an increase in ionic strength, giving some reduction in the zeta potential and a decreased thickness of the diffuse part of the electrical double layer, or
16 •
Z. Bajza et al. / Desalination 171 (2004) 13-20
specific adsorption of counterions to neutralise the particle charge.
In both cases, additives effective for negative particles should be salts with highly charged cations. Among the anions, nitrate has very little tendency to coordinate with metal ions and does not have a significant influence on destabilisation with metal coagulants. However, anions such as bicarbonate, chloride, sulfate, etc., do have considerable effects on the coagulation by ahminium salts. Generally, bicarbonate, sulfate and chloride have little or no effect on the pH of ahminium precipitation. However, they may exert great influence on the range of pH values where the initial precipitate can aggregate to settleable flocs [9]. Sulfate is a moderately strong coordinator with ahminium, and the presence of a sulfate ion extends the pH range of coagulation towards the acid side under normal coagulation condition. Polymeric additives can also be used to cause aggregation of particles, and they may act either by polymer bridging or charge neutralisation (including "electrostaticpatch" effects) [10]. The anionic flocculant is a polyacrylamide emulsion containing anionic and non-ionic flocculas. The charge degree of that flocculant is 30%, specific molecular weight from 0.99-1.03 (at 25°C), and the freezing point is - 18°C. The function ofa polyelectrolyte in the solidaqueous liquid separation process is to overcome the electrocinetic repulsive forces between suspended particles by • charge neutralization-- a hydrophobic colloidal coagulation induced by the direct reduction of the surface charge on the particles, or • bridging - - adsorption of the polyelectrolyte molecule in solution on the surface of two or more suspended particles, joining them together into a network. The anionic flocculant, polyacrylamide, is used for building the size of flocs and has a very
Q ~
-
(
Q ~
~
Like c~arges on
suspenclecl particles repel
Poqmer
Pol ,met 'brid( les"
Floc formation
Fig. 1. "Bridging" model [11]. high molecular weight that is needed to produce large, fast-settling flocs by briding many small primary flocs. The "bridging model" is shown in Fig. 1 [11]. The treatment of tannery wastewater has been a very important issue for pollution control in leather-producing countries due to its heavy pollutant content. However, before carrying out studies to investigate the options for a costeffective treatment of tannery wastewater, it is necessary first to examine the behaviour of the pretreatment [ 12].
2. Materials and methods
2.1. Experimental set-up and sampling The liming process of bovine leather was performed in a leather factory using 150% water, 2.5% Na2S and 3.5% Ca(OH)z. The amounts of chemicals were calculated by kilograms of wetsalted bovine leather, average weight of 35 kg. The tanning process was performed using 200% water, 1.5% (NH4)2804, 1% HCOOH, 0.5%
z. Bajza et al. / Desalination 171 (2004) 13-20
17
CH3-COOH , 6% NaC1 and 6% [Cr(H20)sOH]SO 4. For experimental analysis the wastewaters derived from liming and tanning processes were taken and mixed together in ratio 1:1 (basic waste solution).
by following equation:
2.2. Precipitation
COD was determined using the colorimetric method with a DR100 colorimeter in Hach cuvettes and in the presence of potassium bicarbonate and sulphuric acid. The suspended solids mass was weighed after burning on filter paper at a temperature of 600°C until a constant mass was achieved. Turbidity was measured on a turbidity meter and is expressed as the optical property of omitted and absorbed light. The suspended solids concentration is expressed as a relation of dimension, shape and refraction index of material particles. The following equation performed Nephelometric turbidity units (NTU):
In 200 ml of basic waste solution, for each sample M2(SO4) 3 was added in four different amounts. In sample I the amount of Alz(SO4) 3 added was 0.0032 ml, in sample II 0.0064 ml, in sample II! 0.0128 ml and in sample IV 0.0256 ml. After 5 rain of the mixing process and sedimentation (mud formed in 30 min), the settling velocity was measured, along with pH-value, suspended solids mass, volume of sediment, y(Na2S), y(Cr203) and chemical oxygen demand (COD). Also the turbidity (NTU-method) of supernatant was determined. The same measurements were performed in basic waste solution (before the addition of the A12(SO4)3). The second part of the research was performed using samples I, II, III and IV with the addition of used concentrations of A12(SO4)3. In each of these samples 0.01 ml of anionic polyelectrolyte flocculant was added.
10Cr3+ + 6MnO4 + 11H20 ~ 5Cr202+ 6Mn 2+ + 22H ÷
NTU-
A x (B +C) C
(10)
(11)
where A is the diluted sample turbidity, measured by a turbidity meter, B is the volume of water for dilution (ml) and C is the volume of the sample (ml).
2.3. Methods
The phi was measured using an electronic pHmeter (with standards pH = 4 and 10). The settling velocity was measured in a measuring cylinder for I0, 20 and 30 min. The volume of precipitated sediment was determined as the movement: of the interface through the graduated settling column over time. The concentration of sulphides was determined using the standard titration method with Na2S203 in the presence of iodine. A concentration of chromium, as Cr203, was determined by using the KMnO4 for chromium (III) oxidation, and then the titration method with NazS203 in the presence of KI. Oxidation of chromium (III) in chromium (VI) was performed
3. Results The results are shown in Figs. 2 and 3 and Table 1.
4. Discussion The basic waste solution had high turbidity because of a high concentration of lime, sulphides and organic matter. The high concentration of chromium(III) salts caused the green colour of the solution. The solution contained particles of collagen and keratin obtained after the liming process. The clear-white supematant was separated after the precipitation process with flocculants
18
Z, Bajza et al. / Desalination 171 (2004) 13-20
~
~
O 0
O 0
~
~
~
%00
~ 0 e,~
O o~ o' 0-
O 0
O ~
~
O 0
•
~
~
o o
ID
O 0
•
II
II
~ ~
"0
r.n
_=7 O 0
'~
o
~.~ .N r~
÷
o~
+
~c[
=
÷
~c[o~
=
.~c~
+
~
÷
=oo
~ ~
C&
~
,,,
.~=
÷
+
Z. Bajza et al. / Desalination 171 (2004) 13-20 80'i S~ma01eIV0 ~ .
601
Sample Ill • ~ . . ~ . _ ~ Sample 11• . . . . . . . ~ - -
"~-~-~•~~___ ~----~
• . . . .
~--•
40> 20S,-u'npleI •
1'0
-•. If5
J 20 Time/rain
• ~ 25
30
Fig. 2. Time dependency of sediment volume after A12(SO4)3 precipitation.
60,.2
>
40"
Sample IVe.................... Sample 111• ..... "............. Sample
[l • ....................
............. • . . . . . . . . ........................... • . . . . . . . . . . . "
-............................ ................................. •
19
Suspended solids mass increased with a concentration of added A12(SO4)3. The concentration of Na2S increased more rapidly in samples with the polyelectrolyte with a concentration ofA12(SO4) 3added. The addition of A12(SO4)3 also increased the amount of ChO3 in sludge, but not so drastically. COD value increased with the increase of A12(SO4)3added, which indicates that the precipitation process of organic content is better (higher COD in sludge). The precipitation process of organic content is positively affected by the addition of a polyelectrolyte. The turbidity of the supernatant without the polyelectrolyte was 4472, while with increase of A12(SO4)3 concentration COD value decreased to 8.1. After the addition of A12(SO4) 3 the initial turbidity value was considerably lower, and in sample IV that value decreased to 60.
20Sample
I • ................................................. • ............................................ •
,'0
r
,5
i
20
5
;0
Time/rain
Fig. 3. Time dependency of sediment volume after A12(SO4)3 + polyelectrolyte precipitation.
and coagulants. According to the settling velocity measurement, it can be concluded that the precipitation process was finished after 30 min. After precipitation using A12(SO4)3, the pH value decreased (from 8.30 to 7.60), but after adding the polyelectrolyte, that decrease was smaller (from 8.40 to 8.30). Consequently, as pH value decreased, the volume of sediment increased in all samples (with or without the polyelectrolyte). In samples after precipitation with a mixture of A12(504) 3 and polyelectrolyte, the volume of sediment obtained was smaller, but that process formed ticked sludge. The average value of suspended solids mass in a sample with polyelectrolyte was 0.3736 g, and in the sample without polyelectrolyte 0.3611 g.
5. Conclusions
This study indicates that an increased addition of concentration ofA12(SO4) 3resulted in a greater settling velocity and in more acceptable environmental parameters in the supernatant. This is because of the greater concentration of harmful substances in the sludge. With the addition of an anionic polyelectrolyte, the concentration of harmful substances in the sludge grows. Significant is the growth of y(Na2S) in the sludge, which is 4.5 times larger with the addition ofA12(SO4)3; the COD value is increased 1.4 times and the turbidity of the supernatant was reduced 2.25 times. An increase ofA12(SO4)3concentration caused a small increase in the most observed parameters. A significant increase appeared in sludge volume and in smaller turbidity. In both cases, with or without an anionic polyelectrolyte, at lower pH the solution appeared clear, indicating the presence of very small, colloidal particles. As the pH value increased, the particle size increased, giving higher turbidity.
20
~. Baiza et al. / Desalination 171 (2004) 13-20
This investigation may help to determine the coagulant and flocculant concentrations that are necessary for wastewater treatment. The study may also help in designing methods for tannery wastewater purification.
[6]
[7]
References
[8]
[1] E. Heidemann, Fundamentals of Leather Manufacturing, Eduard Roether, Darmstadt, 1993. [2] J. Ludvik, Low-waste leather technologies, paper presented at the Expert Group Meeting on Pollution Control in Tanning Industry in the South-East Asia Region, Madras, India, 1991. [3] K. Pauligk and R. Hagen, Lederherstellung, VEB, Fachbuchverlag, Leipzig, 1987. [4] K.J. Biefikiewicz, Physical Chemistry of Leather Making, Krieg Publishing, Malabar, FL, USA, 1983. [5] A.D. Covington, The 1998 John Arthur Wilson
[9]
[10]
[11] [12]
Memorial Lecture: New tannages for the new millennium. JALCA, 93(6)(1998) 168-183. G.W. van Loon and S.J. Duffy, Environmental Chemistry, Oxford University Press, New York, 2000. M. Rossini, J. Garrido, J. Garcia and M. Galluzzo, Optimization of the coagulation - - flocculation treatment: influence of rapid mix parameters. Water Res., 33(8) (1999) 1817-1826. K. Otthrner, Encyclopedia of Chemical Technology, Wiley, New York, 1980. P.L. Hayden and A.J. Rubin, Aqueous-Environmental Chemistry of Metals, Ann Arbor Science, Ann Arbor, 1974, p. 180. J. Duan and J. Gregory, Coagulation by hydrolysing metal salts. Adv. Coll. Interf. Sci., (100-t 02)(2003) 475-502. D.A. Mortimer, Synthetic polyelectrolytes - - A review. Polymer Internat., 25(1) (1991) 29-41. Z. Song, C.J. Williams and R.G.J. Edyvean, Sedimentation of tannery wastewater. Water Res., 34(7) (2000) 2171-2176.
ELSEVIER
www.elsevier.com/locate/desal
Influence of different concentrations of A12(804) 3 and anionic polyelectrolytes on tannery wastewater flocculation v
Zeljko Bajza*, Petra Hitrec, Marko MuSic Faculty of Chemical Engineering and Technology, Marulidev trg 19, 10000 Zagreb, Croatia TeL +385 (1) 483-3850; Fax +385 (1) 466-7526; email: [email protected]
Received 8 September 2003; accepted 1 April 2004
Abstract
Tarming of hides and skins to convert them into leather can have a considerable environmental impact. Wastewater treatment technology can successfully purify leather wastewater. The first part of this research investigated the influence ofAl2(SO4)3concentration on the velocity of tannery wastewater settling. The wastewater sample was taken from two of the most polluted wastewater flows after liming and after chrome tanning. These wastewater flows were mixed together in a ratio of 1:1. The second part was carried out with the samples of wastewater mixed together with different concentrations of Alz(SO4)3. The behaviour of settling was investigated after an addition of different concentrations of anionic polyelectrolytesin these samples. The pH, suspended solids mass, volume of sediment, y(Na2S), ~,(Cr203), chemical oxygen demand, along with turbidity in the supernatant were determined in the obtained sludge. The results demonstrate the influence ofA12(SO4)3and anionic polyelectrolyte concentrations on the parameters studied and the improvement of environmental properties. Keywords: Tanneries; Wastewater; A12(SO4)3;Anionic polyelectrolyte; Liming; Chrome tanning;Sludge properties
1. Introduction
The liming process is the first major step in leather making. The pelt has to be freed of the epidermis and hair, including the hair roots, and the keratinous material filling the hair follicles. The group of proteins known as keratins differs from collagen, elastin and many other hide pro*Corresponding author.
teins in that they have a much greater content of cystine residues forming inter- and intra-protein crosslinks. These S-S bridges can be split by reduction or oxidation quite selectively without influencing the collagen fibre network. Reduction can be performed by almost any kind ofreductive agents, preferably under alkaline conditions. The most widely used compounds are reductive sulphur- or thio- compounds. They act by ex-
0011-9164/04/5- See front matter © 2004 Elsevier B.V. All rights reserved doi: 10.1016/j.desal.2004.04.003
14
Z,. Baiza et al. / Desalination 171 (2004) 13-20
changing with one of the sulphur atoms in the disulphide bridge of cystine [ 1]. O
O
II
If
---C-HN-CH-C-NH--
II
o
---C-HN-CH-C-NH--'
I
IJ
c.
o
S + 2HS -- R
I O
tJ
(1)
~"
When the ionic strength = 0 at 27°C, the value of k is 2.15+0.15x10 -4 min -l. The measured dissociation constant of the hexaaquo-chromic cation is [3] Ka = [Cr(H20)S(OH) 2+ [H30 ] + [Cr(H20)6]
S--S--R
CIH
o
I
HN - C H - C
c.
(4)
S--S--R
S
-'-C-
I
6k [Cr(H20)6] 3+
II II
-Nil ---
O
"-C-IIN
I
CH
= -NH ---
II
Chrome tanning, as it is commonly called in the leather trade, is the most widely applied tanning process world-wide, accounting for between 70% and 80% of all leather production. The trivalent chromium ion, Cr 3+, has outstanding tanning properties, allowing the manufacture of a wide range of leathers with remarkable mechanical, physical and chemical characteristics as well as high shrinkage temperatures. The chromium salt most frequently used for this purpose is the basic hydrated sulphate, which has the formula [2]: (2)
In terms of space, six ligands take positions on the angles of the octahedron. The charge of that complex, in dissociated form, is 3+. Because of warming, water molecules are pressed back and changed with anions from outside the complex [3]. The question, how strong is the bond between water molecules and chromium in water solution?, can be answered by carrying out an exchange reaction between these ions in ~80-enriched water: [Cr(H20)6] 3+ + H2'80 ~ [Cr(H2Os) (H2'80)] 3+ + H20
(5)
J -CtI-C
O
[Cr (H20)5OH]SO 4
10 -3"8z
3+
The kinetics of chrome tanning is complicated. Relative initial reaction rates for typical limed collagen may be estimated from the Handerson equation: pKa = pH +log {[HAft[A-]}
(6)
if it is assumed that the pKa of the sidechain carboxyls is 4 and that reaction only occurs at ionized carboxyls [5]. Traditional leather tanning processes involve treating the hides with an aqueous solution of chromium. Suppose the wastewater from a tannery contains 26 mg/L chromium, originally in the Cr 3+ state. As the effluent flows downstream, the dissolved oxygen can oxidize Cr 3+ to Cr2072-. We calculated the extent of oxidation for a situation where oxygen in the stream water is in equilibrium with atmospheric oxygen and has a pH of 6.5 (a H30* = 10-6"5). The first step is to calculate the concentrations of Cr 3÷ and Cr2072assuming that the chromium species are also in equilibrium with the system [6]. The relevant reaction for atmospheric 02 in equilibrium with water (often called a well aerated system) is: O2(g ) + 4H30 + (aq) + 4e - ,y----'
6H20
E ° = 1.23 V (3) E° -
The reaction rate for one water molecule is defined by constant k.
1.23 V
0.0591 V
p E ° = 14.1
- 20.8
(7)
Z. Bajza et al. /Desalination 171 (2004) 13-20
For the'. Cr system: Cr202- (aq) + 14H30 + (aq) +6e-
,__'
2Cr3+(aq)+17H20
p E ° = 23.0
(8) l log
pE=pE°--6
[Cr3*]2 [Cr2072-]ta +,,4 H30 ]
Since the chromium and oxygen systems are in equilibrium, p E is the same for both: Cr3+]2
=
1.6 × 10 -38
(9)
This very :smallratio indicates that virtually all of the Cr3+would be oxidized to Cr2072- and this, in fact, has serious environmental consequences [6]. Coagulation/flocculation mechanisms: Coagulation/flocculation followed by clarification is the most widely used process for treating wastewater from tannery processing. The process usually consists of the rapid dispersal of a coagulant into the wastewater followed by an intense agitation commonly defined as rapid mixing. The most widely used coagulants are aluminium(III) and iron(III) salts [7]. Liquid aluminium sulphate is a 49 wt% solution of A12( $ 0 4 ) 3 × 14 H20, or about 8.3 wt% aluminium as AI203 [8]. Aluminium salts are effective in removing a broad range of impurities from water, including colloidal particles and dissolved organic substances. Their mode of action is generally explained in terms of two distinct mechanisms: charge neutralization of negatively charged colloids by cationic hydrolysis products and incorporation of impurities in an amorphous hydroxide precipitate. The relative importance of these mechanisms depends on factors such as pH and coagulant dosage. At around neutral pH, AI(III) has limited solubility because of the precipitation of an amor-
15
phous hydroxide, which can play a very important role in practical coagulation and floeculation processes. More importantly in practice, hydroxide precipitation leads to the possibility of sweep floceulation, in which impurity particles become enmeshed in the growing precipitate and thus effectivelyremoved. The aluminium hydroxide is of very low solubility, and an amorphous precipitate can form at intermediate pH values. This is of enormous practical significance in the action of these materials as coagulants. With a further increase in pH, the soluble anionic form Me(OH)4 becomes dominant. As well as the simple monomeric hydrolysis products discussed above, there are many possible polynuclear forms that can be considered. Mononuclear AI species (Alo) react almost instantaneously and polynuclear species (Alb) much more slowly. The varying charge with pH can greatly affect the precipitation process. At around neutral pH for aluminium, the initially formed colloidal precipitate is positively charged and, hence, is colloidally stable. As the pH is increased towards the IEP, the stability decreases and the particles can aggregate into large, settleable floes. The presence of highly charged anions, such as sulfate, can have a large effect on hydroxide precipitation. Sulfate can reduce the positive charge of the precipitate in the acid region so that large floes are formed over a wider pH range. Over the usual range of natural water pH (say, 5-9) particles nearly always carry a negative surface charge. Because of their surface charge, aquatic particles are often colloidally stable and resistant to aggregation. For this reason, coagulants are needed to destabilise the particles. According to the classical ideas of colloid stability, destabilisation can be brought about by either: • an increase in ionic strength, giving some reduction in the zeta potential and a decreased thickness of the diffuse part of the electrical double layer, or
16 •
Z. Bajza et al. / Desalination 171 (2004) 13-20
specific adsorption of counterions to neutralise the particle charge.
In both cases, additives effective for negative particles should be salts with highly charged cations. Among the anions, nitrate has very little tendency to coordinate with metal ions and does not have a significant influence on destabilisation with metal coagulants. However, anions such as bicarbonate, chloride, sulfate, etc., do have considerable effects on the coagulation by ahminium salts. Generally, bicarbonate, sulfate and chloride have little or no effect on the pH of ahminium precipitation. However, they may exert great influence on the range of pH values where the initial precipitate can aggregate to settleable flocs [9]. Sulfate is a moderately strong coordinator with ahminium, and the presence of a sulfate ion extends the pH range of coagulation towards the acid side under normal coagulation condition. Polymeric additives can also be used to cause aggregation of particles, and they may act either by polymer bridging or charge neutralisation (including "electrostaticpatch" effects) [10]. The anionic flocculant is a polyacrylamide emulsion containing anionic and non-ionic flocculas. The charge degree of that flocculant is 30%, specific molecular weight from 0.99-1.03 (at 25°C), and the freezing point is - 18°C. The function ofa polyelectrolyte in the solidaqueous liquid separation process is to overcome the electrocinetic repulsive forces between suspended particles by • charge neutralization-- a hydrophobic colloidal coagulation induced by the direct reduction of the surface charge on the particles, or • bridging - - adsorption of the polyelectrolyte molecule in solution on the surface of two or more suspended particles, joining them together into a network. The anionic flocculant, polyacrylamide, is used for building the size of flocs and has a very
Q ~
-
(
Q ~
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Fig. 1. "Bridging" model [11]. high molecular weight that is needed to produce large, fast-settling flocs by briding many small primary flocs. The "bridging model" is shown in Fig. 1 [11]. The treatment of tannery wastewater has been a very important issue for pollution control in leather-producing countries due to its heavy pollutant content. However, before carrying out studies to investigate the options for a costeffective treatment of tannery wastewater, it is necessary first to examine the behaviour of the pretreatment [ 12].
2. Materials and methods
2.1. Experimental set-up and sampling The liming process of bovine leather was performed in a leather factory using 150% water, 2.5% Na2S and 3.5% Ca(OH)z. The amounts of chemicals were calculated by kilograms of wetsalted bovine leather, average weight of 35 kg. The tanning process was performed using 200% water, 1.5% (NH4)2804, 1% HCOOH, 0.5%
z. Bajza et al. / Desalination 171 (2004) 13-20
17
CH3-COOH , 6% NaC1 and 6% [Cr(H20)sOH]SO 4. For experimental analysis the wastewaters derived from liming and tanning processes were taken and mixed together in ratio 1:1 (basic waste solution).
by following equation:
2.2. Precipitation
COD was determined using the colorimetric method with a DR100 colorimeter in Hach cuvettes and in the presence of potassium bicarbonate and sulphuric acid. The suspended solids mass was weighed after burning on filter paper at a temperature of 600°C until a constant mass was achieved. Turbidity was measured on a turbidity meter and is expressed as the optical property of omitted and absorbed light. The suspended solids concentration is expressed as a relation of dimension, shape and refraction index of material particles. The following equation performed Nephelometric turbidity units (NTU):
In 200 ml of basic waste solution, for each sample M2(SO4) 3 was added in four different amounts. In sample I the amount of Alz(SO4) 3 added was 0.0032 ml, in sample II 0.0064 ml, in sample II! 0.0128 ml and in sample IV 0.0256 ml. After 5 rain of the mixing process and sedimentation (mud formed in 30 min), the settling velocity was measured, along with pH-value, suspended solids mass, volume of sediment, y(Na2S), y(Cr203) and chemical oxygen demand (COD). Also the turbidity (NTU-method) of supernatant was determined. The same measurements were performed in basic waste solution (before the addition of the A12(SO4)3). The second part of the research was performed using samples I, II, III and IV with the addition of used concentrations of A12(SO4)3. In each of these samples 0.01 ml of anionic polyelectrolyte flocculant was added.
10Cr3+ + 6MnO4 + 11H20 ~ 5Cr202+ 6Mn 2+ + 22H ÷
NTU-
A x (B +C) C
(10)
(11)
where A is the diluted sample turbidity, measured by a turbidity meter, B is the volume of water for dilution (ml) and C is the volume of the sample (ml).
2.3. Methods
The phi was measured using an electronic pHmeter (with standards pH = 4 and 10). The settling velocity was measured in a measuring cylinder for I0, 20 and 30 min. The volume of precipitated sediment was determined as the movement: of the interface through the graduated settling column over time. The concentration of sulphides was determined using the standard titration method with Na2S203 in the presence of iodine. A concentration of chromium, as Cr203, was determined by using the KMnO4 for chromium (III) oxidation, and then the titration method with NazS203 in the presence of KI. Oxidation of chromium (III) in chromium (VI) was performed
3. Results The results are shown in Figs. 2 and 3 and Table 1.
4. Discussion The basic waste solution had high turbidity because of a high concentration of lime, sulphides and organic matter. The high concentration of chromium(III) salts caused the green colour of the solution. The solution contained particles of collagen and keratin obtained after the liming process. The clear-white supematant was separated after the precipitation process with flocculants
18
Z, Bajza et al. / Desalination 171 (2004) 13-20
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Z. Bajza et al. / Desalination 171 (2004) 13-20 80'i S~ma01eIV0 ~ .
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Sample Ill • ~ . . ~ . _ ~ Sample 11• . . . . . . . ~ - -
"~-~-~•~~___ ~----~
• . . . .
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Fig. 2. Time dependency of sediment volume after A12(SO4)3 precipitation.
60,.2
>
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Sample IVe.................... Sample 111• ..... "............. Sample
[l • ....................
............. • . . . . . . . . ........................... • . . . . . . . . . . . "
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19
Suspended solids mass increased with a concentration of added A12(SO4)3. The concentration of Na2S increased more rapidly in samples with the polyelectrolyte with a concentration ofA12(SO4) 3added. The addition of A12(SO4)3 also increased the amount of ChO3 in sludge, but not so drastically. COD value increased with the increase of A12(SO4)3added, which indicates that the precipitation process of organic content is better (higher COD in sludge). The precipitation process of organic content is positively affected by the addition of a polyelectrolyte. The turbidity of the supernatant without the polyelectrolyte was 4472, while with increase of A12(SO4)3 concentration COD value decreased to 8.1. After the addition of A12(SO4) 3 the initial turbidity value was considerably lower, and in sample IV that value decreased to 60.
20Sample
I • ................................................. • ............................................ •
,'0
r
,5
i
20
5
;0
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Fig. 3. Time dependency of sediment volume after A12(SO4)3 + polyelectrolyte precipitation.
and coagulants. According to the settling velocity measurement, it can be concluded that the precipitation process was finished after 30 min. After precipitation using A12(SO4)3, the pH value decreased (from 8.30 to 7.60), but after adding the polyelectrolyte, that decrease was smaller (from 8.40 to 8.30). Consequently, as pH value decreased, the volume of sediment increased in all samples (with or without the polyelectrolyte). In samples after precipitation with a mixture of A12(504) 3 and polyelectrolyte, the volume of sediment obtained was smaller, but that process formed ticked sludge. The average value of suspended solids mass in a sample with polyelectrolyte was 0.3736 g, and in the sample without polyelectrolyte 0.3611 g.
5. Conclusions
This study indicates that an increased addition of concentration ofA12(SO4) 3resulted in a greater settling velocity and in more acceptable environmental parameters in the supernatant. This is because of the greater concentration of harmful substances in the sludge. With the addition of an anionic polyelectrolyte, the concentration of harmful substances in the sludge grows. Significant is the growth of y(Na2S) in the sludge, which is 4.5 times larger with the addition ofA12(SO4)3; the COD value is increased 1.4 times and the turbidity of the supernatant was reduced 2.25 times. An increase ofA12(SO4)3concentration caused a small increase in the most observed parameters. A significant increase appeared in sludge volume and in smaller turbidity. In both cases, with or without an anionic polyelectrolyte, at lower pH the solution appeared clear, indicating the presence of very small, colloidal particles. As the pH value increased, the particle size increased, giving higher turbidity.
20
~. Baiza et al. / Desalination 171 (2004) 13-20
This investigation may help to determine the coagulant and flocculant concentrations that are necessary for wastewater treatment. The study may also help in designing methods for tannery wastewater purification.
[6]
[7]
References
[8]
[1] E. Heidemann, Fundamentals of Leather Manufacturing, Eduard Roether, Darmstadt, 1993. [2] J. Ludvik, Low-waste leather technologies, paper presented at the Expert Group Meeting on Pollution Control in Tanning Industry in the South-East Asia Region, Madras, India, 1991. [3] K. Pauligk and R. Hagen, Lederherstellung, VEB, Fachbuchverlag, Leipzig, 1987. [4] K.J. Biefikiewicz, Physical Chemistry of Leather Making, Krieg Publishing, Malabar, FL, USA, 1983. [5] A.D. Covington, The 1998 John Arthur Wilson
[9]
[10]
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Memorial Lecture: New tannages for the new millennium. JALCA, 93(6)(1998) 168-183. G.W. van Loon and S.J. Duffy, Environmental Chemistry, Oxford University Press, New York, 2000. M. Rossini, J. Garrido, J. Garcia and M. Galluzzo, Optimization of the coagulation - - flocculation treatment: influence of rapid mix parameters. Water Res., 33(8) (1999) 1817-1826. K. Otthrner, Encyclopedia of Chemical Technology, Wiley, New York, 1980. P.L. Hayden and A.J. Rubin, Aqueous-Environmental Chemistry of Metals, Ann Arbor Science, Ann Arbor, 1974, p. 180. J. Duan and J. Gregory, Coagulation by hydrolysing metal salts. Adv. Coll. Interf. Sci., (100-t 02)(2003) 475-502. D.A. Mortimer, Synthetic polyelectrolytes - - A review. Polymer Internat., 25(1) (1991) 29-41. Z. Song, C.J. Williams and R.G.J. Edyvean, Sedimentation of tannery wastewater. Water Res., 34(7) (2000) 2171-2176.