INTRODUCTION
The discharge of wastes, particularly the industrial wastes into the
water bodies has been a matter of great concern in the industrialized
countries of the world since long. With the rapid growth of industries
in India, pollution has increased tremendously.
Tanning industry is one of the important industries in India, which earns
considerable foreign exchange through the leather export. There are about
5000 tanneries in India. The quantity of effluent released from the tanneries
is about 50 to 60 L kg-1 of leather tanned. The tannery wastes
are ranked as high pollutants among the industrial wastes. Tannery effluent
is rich in salt content especially the chromium. Chromium in its hexavalent
form is one of the undesirable heavy metals because it affects human physiology,
accumulates in the food chain and causes several ailments (Park and Jung,
2001).
The trivalent form is relatively innocuous, but hexavalent chromium is
toxic, carcinogenic and mutagenic in nature, highly mobile in soil and
aquatic system and also is a strong oxidant capable of being adsorbed
by the skin (Singh and Singh, 2002). So the removal of Cr (VI) besides
colour, sodium, calcium, magnesium salts from tannery effluents is important
before discharging them into aquatic environments or on to land.
A wide range of physical and chemical processes are available for the
removal of Cr (VI) and salts from tannery effluents include chemical precipitation,
reverse osmosis, evaporation, ion exchange and adsorption. Adsorption
on Activated Carbon (ARC) has been adopted as tertiary treatment in various
types of industries because of its excellent adsorption capability (Bailey
et al., 1999). However, it's use is limited by its high cost (El-Geundi,
1997). In this context, vermiculite mineral with its high cation exchange
capacity and reactive surface area was scientifically evaluated for its
potential to substitute the activated carbon which could be cost effective
and economically feasible treatment method. Column studies were carried
out to assess the suitability of raw and exfoliated vermiculite grades
obtained from Tamil Nadu Minerals Ltd., Chennai for the removal of Cr
(VI) and specific pollutants from tannery effluent.
MATERIALS AND METHODS
The untreated chrome tan liquor collected from a tannery at Erode, Tamil
Nadu was analysed for its physical and chemical properties following the
standard procedures.
Raw vermiculite comprising of aluminium, iron, magnesium, silicate mineral
mixtures, which is excavated as a mineral comprising of thin layers was
obtained from M/s. TAMIN, Chennai. Exfoliated vermiculite is obtained
by heating the raw vermiculite to temperatures upto 1000°C. These
vermiculite are graded accordingly to specific sizes.
Mono and mixed vermiculite columns: Mono and mixed column experiments
using selected raw (RVG) and Exfoliated Vermiculite Grades (EVG) were
carried out using PVC pipes of 5 cm diameter and 50 cm long (height).
The bottom of the pipe was fitted with a filter paper (Whatman No. 1)
and a wire mesh (0.1 mm), which were tightly wrapped to hold the weight
of different grades of vermiculite. At the bottom, the treated effluent
(leachate) was collected through a funnel.
Four selected grades of vermiculites (RVG 2, 3, 4 and EVG 5) were gently
packed in the PVC pipes of required height in the column which exhibited
varied bulk densities ranging from 0.95 to 1.48 g cc-1 with
four replications. Four treatments with mixed grades of vermiculite (RVG2
mixed with equal quantities of RVG3 (T1). RVG4
(T2), EVG (T3) and RVG5 (T4))
were also gently packed to required column height which exhibited varied
bulk densities ranging from 1.61 to 1.98 g cc-1.
Calculation of pore volume: To calculate one pore volume, the
weight of the columns packed with mono and mixed grades of vermiculites
saturated with tannery effluent was substracted from its original weight.
The columns were leached based on pore volumes and leachates collected
from four pore volumes were analyzed for pH, EC, TS, chromium (VI), sodium,
sulphate, calcium and magnesium following standard methods.
RESULTS AND DISCUSSION
The analytical results of the chrome tan liquor are furnished in Table
1. The effluent was neutral in reactions with high EC (20.3 dS m-1),
TS (12871 mg L-1), colour (3.15 OD unit), total chromium (206
mg L-1), sodium (2816 mg L-1), sulphate (1496 mg
L-1), calcium (216 mg L-1) and magnesium (123 mg
L-1). The composition of chrome tan liquor mainly depended
on the chemicals present in hides, products formed during the decomposition
and chemicals used in the tanning of the hides.
| Table 1: |
Physico-chemical characteristics of raw tannery effluent |
|
| Table 2: |
Physical and chemical properties of different grades
of raw and exfoliated vermiculites |
|
| (Mean of three replications); RVG-Raw vermiculite grade;
EVG-Exfoliated vermiculite grade |
Table 2 records the analytical results of the both
the raw and exfoliated vermiculite grades. These adsorbent materials were
alkaline in nature which could be due to the presence of associated carbonate
rock impurities, the reaction of which is normally alkaline. The bulk
density, total surface area, particle density and CEC were the highest
in RVG2.
The trend of retention of soluble salts and removal of colour by mono
(0.73 v/v) and mixed columns (1.52 v/v) (Table 3 and
4) was a maximum adsorption during the first pore volume,
followed by a linear decreasing trend upto the third pore volumes. In
the fourth pore volume, the adsorption was the least. There was only a
meager difference with reference to pollutant load between the leachate
of fourth pore volume compared to that of the original values, indicating
that the vermiculite at this stage has attained the point of saturation
of all its exchange sites.
Among mono columns, reduction of total solids was maximum in RVG2
(T1) which was found to be 51.1% in the first pore volume,
48.1, 35.3, 5.34% in the 2nd, 3rd and 4th pore volumes. But incase of
mixed columns, 50% RVG2 + 50% RVG3 columns (T1)
was found to reduce total solids by 52.8, 51.5, 37.8 and 8.13% in the
1st, 2nd, 3rd and 4th pore volumes. The differences in the per cent removal
of total solids among mono and mixed column could be ascribed to the increased
bulk density of the latter system than the former, besides the enhanced
CEC.
The percentage reduction of total solids which decreased with the advancement
of pore volumes indicated that the predominant sorption mechanism initially
was probably physical sorption associated with the Van Der Waals force
which consequently would have been reversed due to weak adsorption as
the pore volume advanced. A similar result was observed by Sasi Kala Rani
(2003) who reported that vermiculite layer showed greater percent of removal
in the first three pore volumes and the adsorption decreased thereafter
with reference to soluble barium and salts.
| Table 3: |
Characteristics of vermiculite mixed-columns leachates
of raw tannery effluent |
|
| Table 3: |
Characteristics of vermiculite mixed-columns leachates
of raw tannery effluent |
|
| Table 3: |
Characteristics of vermiculite mixed-columns leachates
of raw tannery effluent |
|
| Table 4: |
Characteristics of vermiculite mono-columns leachates
of raw tannery effluent |
|
| Table 4: |
Characteristics of vermiculite mono-columns leachates
of raw tannery effluent |
|
The same trend was also noticed in case of colour removal where RVG2
(T1) recorded 73% removal in the first pore volume in mono-column
and in mixed column of 50% RVG2 + 50% RVG3 colour
removal of as high as 80.3% was observed. This might be due to the retention
of total solids on the inter-lattice surfaces of vermiculite which resulted
in the colour reduction. Similar observation was also reported by Sumathi
(1999 and 2003).
In mono-columns, the chromium removal was the highest in RVG2
(T1) where it was 63.6% in the first pore volume but incase
of mixed-columns of 50% RVG2 + 50% RVG3, still a
higher quantum removal of chromium 74.6% in the first pore volume was
evident. Incase of removal of cations, the vermiculite mineral in the
columns retained most of the cations viz., Ca, Mg, Na and K which was
evident from the substantial decrease in the concentration of these ions
in the leachate of the first pore volume. The increased adsorption of
chromium and cations at varying pore volumes could be attributed to the
high cation exchange capacity of RVG2 (112 Cmol (p)+
kg-1) and mixed column (RVG3-104 Cmol (p)+
kg-1). These adsorption sites could have adsorbed large amounts
of Cr and strongly retained in the exchange sites. Similar observations
have been reported by Sumathi (1999) and Sasi Kala Rani (2003).
CONCLUSIONS
The efficacy of chromium removal was maximum of 74.6% in the 1st pore
volume of mixed vermiculite column (RVG2 + RVG3),
whereas among the mono columns, it was the highest in RVG2,
were it was 63.6% in the 1st pore volume. The general trend with reference
to the retention and removal efficiency of various cations, anions and
chromium by mono and mixed vermiculite columns was of maximum removal
during the first pore volume followed by a linear decreasing trend upto
third pore volumes and in the fourth pore volume the adsorption decreased
drastically as a result, the leachate of fourth pore volume was all most
equal to that of the original effluent with reference to the pollutant
load. Hence it is inferred that RVG2 + RVG3 mixed
vermiculite column could be utilized as a substitute for activated carbon
in tertiary treatment of tannery effluent.
ACKNOWLEDGMENT
The authors are thankful to Managing Director, Tamil Nadu Minerals Limited,
Chennai for rendering financial support for this research under TNAU-TAMIN
Scheme project.
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