BIOSORPTIVE REMOVAL OF HEAVY METALS FROM WASTEWATER USING DUCKWEED

Water pollution has been recognized as a problem for decades. The use of heavy metals in industries and their regular mining increases their concentration in water bodies. Unlike organic compounds, metals cannot degrade, and therefore effective cleanup requires their immobilization to reduce or remove toxicity. A few conventional methods employed to remove heavy metals from wastewater are expensive, require skilled labors and maintenance. Therefore, the use of aquatic plants has come up since the last few decades. Duckweed is one such plant employed as a biosorbent and has been considered a better alternative than any other aquatic plant because of high tolerance to cold than water hyacinth, more easily harvested than algae, capable of rapid growth (0.1 to 0.5 g g day) and small size of plant. This study aims to determine the suitability of this plant for biosorbing toxic heavy metals commonly found in industrial wastewater, domestic wastewater, and seepage water.


Introduction
We all depend on earth's ecological and atmospheric balance for survival. But the increasing industrialization and urbanization has affected this balance by the introduction of pollutants. These hazardous pollutants consist of a variety of organic compounds and heavy metals, which pose serious risks to human health. Heavy metals are primarily a concern because they cannot be destroyed by degradation 1 . The most common heavy metals at hazardous waste sites are Cadmium (Cd), Chromium (Cr), Copper (Cu), Lead (Pb), Mercury (Hg), Nickel (Ni) and Zinc (Zn). Of these, lead and mercury are two of the most significant contaminants, posing serious and sometimes life threatening health hazards 2 .
The potential use of plants to remediate contaminated soil and groundwater has recently received a great deal of interest. Up to this point the most widely used wastewater treatments such as trickling filters have been mechanical 3 . Such plants have proven to be too complex, costly, and energy-intensive. Wastewater treatment systems which are simple and require little or no maintenance must be developed to fulfill these needs. It is becoming evident that aquatic plants have great potential for wastewater treatment and reclamation. These plants can absorb nitrate, phosphate, heavy metals such as manganese, and other chemical compounds. They are generally used to provide secondary treatment of effluents, in small lagoons filled with cattails or totora 4, 5 . There are now more than fifteen aquatic plant wastewater IJBAR (2011) 02(08) www.ijbar.ssjournals.com treatment systems currently in operation such as Duckweed, Water lettuce, Cattails, Water spinach, Water ferns, Aquatic mosses and liver worts 6 . Duckweed appears to be a better alternative and have been recommended for wastewater treatment as they are more tolerant to cold, easily harvested, has rapid growth rate and small size. This plant floats on or just beneath the surface of still or slow-moving water. It has properties such as adsorbing toxins, heavy metals, nitrates and phosphates, controlling mosquito breeding, reducing water evaporation and removes carbon dioxide from the atmosphere, filtering out bacteria and reducing lightgenerated growth of photoautotrophic algae 6 .

2.
Conventional methods for treatment: The commonly used procedures for removing metal ions from aqueous streams include chemical precipitation, lime coagulation, ion exchange, reverse osmosis and solvent extraction. The process description of each method is presented below 7, 8 . 2.1. Reverse Osmosis: It is a process in which heavy metals are separated by a semi-permeable membrane at a pressure greater than osmotic pressure caused by the dissolved solids in wastewater. The disadvantage of this method is that it is expensive.

Electro dialysis:
In this process, the ionic components (heavy metals) are separated through the use of semipermeable ion selective membranes. Application of an electrical potential between the two electrodes causes a migration of cations and anions towards respective electrodes. Because of the alternate spacing of cation and anion permeable membranes, cells of concentrated and dilute salts are formed. The disadvantage is the formation of metal hydroxides, which clog the membrane.

Ultra filtration:
They are pressure driven membrane operations that use porous membranes for the removal of heavy metals. The main disadvantage of this process is the generation of sludge.

Ion-exchange:
In this process, metal ions from dilute solutions are exchanged with ions held by electrostatic forces on the exchange resin. The disadvantages include: high cost and partial removal of certain ions.

Chemical
Precipitation: Precipitation of metals is achieved by the addition of coagulants such as alum, lime, iron salts and other organic polymers. The large amount of sludge containing toxic compounds produced during the process is the main disadvantage. Hence the disadvantages like incomplete metal removal, high reagent and energy requirements, generation of toxic sludge or other waste products that require careful disposal has made it imperative for a cost-effective treatment method that is capable of removing heavy metals from aqueous effluents 7 . through metabolically mediated or physico-chemical pathways of uptake. Algae, bacteria and fungi and yeasts have proved to be potential metal biosorbents. The major advantages of biosorption over conventional treatment methods include low cost, high efficiency, minimization of chemical and /or biological sludge, No additional nutrient requirement, regeneration of biosorbent and possibility of metal recovery.

Using duckweed as a biosorbent
Nutrient removal from wastewater prevents eutrophication from occurring downstream where the wastewater is discharged into water bodies such as rivers and reservoirs. One nutrient removal system that has been researched extensively over the past 40 years utilizes duckweed plants (Lemnaceae). Duckweed (botanically known as Lemnaceae) is a stem less, aquatic flowering plant. Duckweed is a small and free floating and grows on the surface of still or slow moving water in carpet-like groups. Common Duckweed has 1 to 3 leaves measuring 1/16 to 1/8 inches in length. 1 to 6 roots may grow from each plant. Duckweeds are Stemless and seed bearing plant. There are several varieties of Duckweed that grow together in dense colonies 10 . Duckweed systems rely on three basic principles: nutrient uptake, harvesting, and solids management 9 .
Duckweed grows naturally in almost every region with a growing season of at least five months. Duckweed is a monocot; it floats on water, and has one of the fastest growth rates of any of the macrophytes. Duckweed is the common name for the Lemnaceae family of plants, with species like Lemna minor, Lemna Gibba, Spirodela Polyrhizza, and Wolffia (genus name) 9 . Generally reproduction rate of duckweed is marvelous as they grow twenty times faster than a corn and are able to cover 1 acre in just 45 days if unrestrained 11 . Fresh duckweed fronds contain 92 to 94 percent water. Fiber and ash content is higher and protein content lower in duckweed colonies with slow growth. The solid fraction of a wild colony of duckweed growing on nutrient-poor water typically ranges from 15 to 25 % protein and from 15 to 30 % fiber. Duckweed grown under ideal conditions and harvested regularly will have a fiber content of 5 to 15 % and a protein content of 35 to 45 %, depending on the species involved. Data were obtained from duckweed colonies growing on a wastewater treatment lagoon and from a duckweed culture enriched with fertilizer. Duckweed protein has higher concentrations of the essential amino acids, lysine and methionine, than most plant proteins and more closely resembles animal protein in that respect 12 . Duckweed can be used as a IJBAR (2011) 02(08) www.ijbar.ssjournals.com water purifier. The profitable characteristics of duckweed are high productivity, high protein content, wide geographic distribution and control of negative impacts from conventional wastewater treatment ponds 13 .

The mechanism
The plants are grown hydroponically (in the absence of soil). Their roots, which are home to large numbers of bacteria and other microorganisms, extend into the wastewater. These microorganisms feed off the minerals and organic chemicals that pollute the wastewater. While digesting the pollutants, the microorganisms produce by-products such as sugars and amino acids, which are absorbed by the plant roots as food 9, 14 . The plants in turn supply oxygen and low levels of nutrients to the microorganisms for their rapid growth. This mutually beneficial, or symbiotic, relationship allows wastewater to be purified by the plant roots, and the plants' abundant new leaves help restore oxygen to the air and regulate the level of carbon dioxide and other atmospheric gases. Using aquatic plants is thus an excellent solution for wastewater and for nonindustrial sewage in particular 14

Metals being removed by duckweed
The discharge of heavy metals and other pollutants into aquatic ecosystems has become a matter of concern in India over the last few decades. These pollutants are introduced into the aquatic systems significantly as a result of various industrial operations. The pollutants of concern include Copper, Lead, Zinc, Cadmium, Total suspended solids, Nitrates and Phosphates. These toxic materials may be derived from mining operations, refining ores, sludge disposal, fly ash from incinerators, the processing of radioactive materials, metal plating, or the manufacture of electrical equipment, paints, alloys, batteries, pesticides or preservatives. Over the few decades, several researchers have tried to find the efficiency of duckweed in removing these pollutants. 6.1. Copper: Various studies have been done on the role of Duckweed in copper removal from wastewater. In Egypt, its 100% removal was seen in a study testing Duckweed as an alternative cost effective natural biological tool in wastewater treatment by Wafaa Abou El-Kheir et al. (2007) 18 . In a different study in Algeria by N. Khellaf 19 . In another study with Bioremoval of lead from water using Duckweed, North Dakota, USA, exposed to a single dose of lead (from Pb(NO 3 ) 2 ) at a concentration of 5.0 mg/l for a time period of 21 days. Viable biomass removed 85-90% of the lead, viable duckweed previously exposed to lead removed 70-80% of the lead, non-viable biomass (control group) removed 60-75% of the lead, and there was no removal in the 'no-biomass' control group. Based on these results it was concluded by Gazi Nazmul Haq Rahmani and Steven P. K. Sternberg (1999) that the viable biomass is effective in removing lead present at sub-lethal levels 20 . 6.3. Zinc: Excessive absorption of zinc suppresses copper and iron absorption. The free zinc ion in solution is highly toxic to plants, invertebrates, and even vertebrate fish 35 . In Algeria, The ability of the duckweed Lemna gibba to remove zinc from water was investigated in a quarter coïc solution containing 18 mg/L of Zn 2+ supplied such as zinc sulphate (ZnSO 4 ). The Duckweeds reduced Zn concentration to 0.3 mg/L which is below the safety limit set by the US Environmental Protection Agency (EPA) as shown by N. Khellaf and M. Zerdaoui (2010) 22 . Similarly, the uptake of zinc (Zn) by the duckweed Lemna gibba L., native to the north-east region of Algeria, was investigated in quarter Co c solutions enriched with 6.0, 10.0, 14.0 and 18.0 mg l (-1) of Zn supplied as zinc sulphate (ZnSO 4 18 . In Mexico, Duckweed was exposed during 7 days to Cadmium, and lead, at concentrations ranging from 50 to 300 mg/L in a greenhouse with controled photoperiod and temperature. According to Miranda, M. G. Ouiroz, A. and M. Salazar (2000), there was more than 50% decrease in chlorophyll content in all tests in relation with control 19 . Turkey was not far behind as Cadmium tolerance was investigated under hydroponics conditions. Within a span of 4 days, the plant was capable of removing about 75-85% Cd from 100 mL of both kinds of wastewaters as seen by Yeşim Kara and Izzet Kara (2005) 24 . As the potential dangers of heavy metal pollution in aquatic environments is well known, it is very important the treatment of wastewater. Effluents IJBAR (2011) 02(08) www.ijbar.ssjournals.com containing these metals may be treated by continuously passing them through a bed of these plants growing in ponds. 6.5. Total suspended solids: As levels of TSS increase, a water body begins to lose its ability to support a diversity of aquatic life. Suspended solids absorb heat from sunlight, which increases water temperature and subsequently decreases levels of dissolved oxygen 25 . A decrease from 200 to 300 mg/L in influent to less than 10mg/L in effluent was seen in The LemnaŽ System which treats municipal and industrial wastewater using an aquaculture (aqua farming) method that relies on the growth of duckweed to reduce concentrations of BOD, TSS, and inorganic contaminants 26 . 6.6. Nitrates and Phosphates: When Nitrates and Phosphates are added to an aquatic system through fertilizers or sewage, Eutrophication takes place due to which hypoxia occurs. This induces reductions in specific fish and other animal populations 28 . In a study for Nitrogen removal in recirculated duckweed ponds system in France (2007), two pilot scales recirculated Duckweed-based Ponds were employed to treat municipal wastewater. The average removal efficiencies for TN, TKN and NH 4 -N were 75%, 89% and 92%, respectively at TN loading of 1.3 g/m 2 .d and were 73%, 74% and 76%, respectively at TN loading of 3.3g/m 2 d 29 . Another work in Israel showed Nitrogen removal and conversion by duckweed grown on waste-water. Dry yield of the duckweed approached 15 g m −2 day −1 with a protein content of about 30% in the short retention-time treatments as seen by Gideon Oron et al. (2003) 30 . Phosphate is a limiting factor and must be removed from the effluent before being discharged. In Turkey, The capacity of duckweed in phosphate removal from secondary effluents was studied in laboratory to understand the mechanism of biological phosphate uptake.
Orthophosphate can be efficiently removed if duckweed is frequently harvested. According to E.
Öbek and H. Hasar, (2002) the initial phosphate concentration decreased from 15 mg L-1 to 0.5 mg L-1 at the end of an 8 days period 31 . Duckweed plants typically contain more phosphorus in its tissue than other floating plants, which makes them suitable for phosphorus removal. According to the Wellsville (UT) Municipal Sewage Lagoons studies (2010), while 1%-P is very common in oven dried duckweed, values have been reported from 0.3 up to 2.6%-P. Percent dry matter ranges from 5.4-8% with 69-86% being the organic (volatile) fraction. The N: P ratio is typically 5:1 32 . It shows that duckweed is active to clean the environment from the wastewater. Also, duckweed is found to be a promising adsorbent for the removal of metal cations from mixed metal ions solution, representing an effective and environmentally clean waste matter. Harvesting is an essential component of duckweed nutrient removal systems because it physically removes nutrients from the system via the biomass. Without harvesting, the plant tissue would die, settle to the bottom of the lagoon, decompose and then release the nutrients back into the water column. This harvested biomass can be used as compost, fodder rich in protein, or to generate fuel like methane 9 .