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STUDY OF THE HEAVY METAL POLLUTION TREATMENT
POTENTIAL
OF THE COAL GENERATED HUMIC ACID |
Sumayya Saied, Azhar Siddique, Majid Mumtaz
and Kazim Ali
Department of Chemistry, University of Karachi, Karachi-75270, Pakistan
|
| ABSTRACT |
| Humic acid is a naturally occurring polymeric
compound, which shows high affinity towards heavy metals to form complexes
and the degree of affinity varies with the source and type of humic
acid. In present study humic acid was regenerated from Lakhra lignite
coal source and studied for its metal chelating capability. It is
revealed from the study that the humic acid chelates most strongly
with M2+ and the reactivity order for metal chelation is Pb2+ >
Cu2+ >Fe2+ > Ni2+ > Zn2+ > Cr3+ |
| Keywords: Humic acid, wastewater, heavy metal pollution,
formation constant |
| |
| INTRODUCTION |
Humates are highly biodegraded and compressed remains of ancient
plant and animal minerals. Simply they are highly decomposed organic
materials that have fossilized over a period of millions of years.
They are also referred to as Humic substances and are used as soil
conditioners, soil supplements and fertilizers amendments. Humic acid
is a naturally occurring polymeric organic compound. (Schnitzer, 1972),
designated by nature to perform a wide variety of functions. It is
produced through the decay or oxidation of vegetable matter through
microbial action and is naturally found in soil, rivers, and oceans
and
in lignite coals (Lawson, 1989; Barna, 1983). This wonderful naturally
occurring product is found in abundance in low rank lignite coals
like the ones found
in Pakistan, making Pakistan coals most suitable for Humic acid
production. Humic acid are formed in lignites by a slow decay of
vegetable matter over a period of
thousands of years, by a process called humification (Linehan, 1978).
This process can be visualized as a slow progressive oxidation of
the plant residue under microbial action leading to the formation
of Humic acid. As coal matures, Humic acids gradually lose the mobility
of their oxygen functional groups through complex condensation reactions
are cooperated into coal as insoluble humates. This is the reason
that low rank lignite coals, like the ones found in Pakistan, often
contains high levels of humic acids, which gradually disappears with
the increasing rank of coal (Stefanova, 1993). In soils, humic acid
acts as a plant growth promoter, nutrient carrier (Sina, 1978) and
a soil conditioner (Hans, 1989), perfoliating beneficial soil organism
(Bhardwaj, 1970). In water it kills harmful bacteria and act as an
ion exchange resin (Zak, 1986), removing heavy metals that are harmful
to aquatic life. Due to its antibacterial/antifungal and other therapeutic
properties (Hermani, 1990) it is utilized in veterinary medicines
and as feed additive to promote growth. In industry it carries wide
applications as a pigment, coloring agent and in cosmetics etc (Gelszier
et al., 1981; Barron, 1981). The process of metal absorption by lignites
under laboratory condition, elucidating the role that low rank humic
coals can play in the retention and accumulation of heavy metal. Since
the last century, many observations on retention and accumulation
of metals in fossils organic sediments have been reported. Coals are
one of the largest deposits of fossils organic matter where metal
accumulation has been also observed, mainly in peats and lignites.
Present study was focused to evaluate the potential of lignite coal
regenerated humic acid capability to chelate selected heavy metals
and their use as metal recovery agent from waste water of various
industrial effluents.
|
| MATERIALS AND METHODS |
Humic acid was obtained from Fuel Research Centre, PCSIR, Karachi
complex, all other reagents were of analytical grade purchased from
E-Merck, Germany. Atomic absorption standards were purchased from
Fisher Scientific, UK. Different concentration solutions ranged from
0.01 M to 0.20 M of heavy metals (Cu2+, Zn2+,
Ni2+, Pb2+, Cr3+ and Fe2+)
were prepared from respective analytical grade salt. Two grams humic
acid solublize completely in 100ml of water. The complexing capacity
of humic acid for each metal ion was determined by adding 5 ml of
humic acid solution, 5 ml of 0.1M NaCl and 10 ml of a metal ion solution
to give an excess of metal ion to humic acid. The solution was adjusted
to desired pH (5.0, 5.5, 6.5, 7.5, 8.5) equilibrate for 24 hour at
room temperature and centrifuged to separate the metal-humate. The
supernatant was analyzed by AAS. Each determination was carried out
in triplicate. The number of equivalents (x) of complexing agent combined
with a particular metal ion was found from the slope of the linear
function:
|
| RESULTS |
The observations reveal that humic acid can chelate heavy metals
effectively and have potential to be used in wastewater treatment
for heavy metal removal. It is found that humic acid-metal interaction
is pH dependent, therefore, particular pH was set for each of the
metal. The Cu2+, Fe2+ and Zn2+ showed the maximum interaction at pH=
5.5 (table1), whereas the other metals showed the high affinity at
pH range 6.5 to 7.5 (Fig.1). The study also confirmed that humic acid
chelates more strongly with metals in +2 oxidation state as it is
evident in case of Cr3+ except in case of Zn2+ both were chelated
only up to 30% of the total metal concentration.
|
| DISCUSSION |
Present study reveals the reactivity order for metal-humic acid
chelation is as follows:
Pb2+> Cu2+>Fe2+ > Ni2+>
Zn2+> Cr3+
This study leads to the conclusion that humic acid can be used in
leaching out a metal ion from its aqueous solution hence it can
be used as metal detoxification agent for industrial and domestic
effluents containing high levels of such heavy metals. The complex
formation ability of humic acid also promotes retention and accumulation
of heavy metals for plant growth as humic acid play an important
role in plant’s nutrition because the plants are provided
required micro nutrients or essential trace elements through complex
formation by humic acid. Further studies can be performed to explore
the exact mechanism working behind these observations.
Table 1. Formation constants of metal ion-humic acid complexes (k)
derived from the number of equivalents of humic acid per mol of
metal (x), and complexing capacities.
|
| REFERENCES |
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