Studies of Sorbent Efficiencies of Maize Parts in Fe(II) Removal from Aqueous Solutions

. The efficiency of the cob, sheath, seed chaff and stalk of maize plant in the removal of Fe(II) from aqueous solutions was studied. FTIR analysis of biomass surfaces before and after adsorption showed that seed chaff has the highest number of functional group coordination points. The percentage removal of Fe(II) increased with increase in pH for all the biomass parts with highest efficiency shown by the seed chaff at all the studied pH values. Metal up-take also increased with increase in seed chaff load. This direct relationship was however not shown by other parts where decreases in metal up-take were observed at high doses of the biomass. At optimum pH and biomass load, equilibrium adsorption capacities were reached in 30 minutes for all the parts. The efficiency of the biomass parts in the sorption process were in the order seed chaff>stalk>sheath>cob. At optimum conditions of the study, the seed chaff removed 73 % of Fe(II) from solution in its natural state.


Introduction
Water pollution by heavy metals has been a major problem in recent times due to the rapid industrialization of the modern world. With the growth of industrial activities, contamination of the environment with wastewater and heavy metals has become a major problem [1]. Iron ion is an important corrosion product that is commonly found in fluids of industrial discharges. In drinkingwater supplies, iron(II) salts are unstable and are precipitated as insoluble iron(III) hydroxide which settles out as a rust-coloured silt. Anaerobic ground waters may contain iron(II) at concentrations of up to several milligrams per litre without discolouration or turbidity in the water when directly pumped from a well, although turbidity and colour may develop in piped systems at iron levels above 0.05 -0.1 mg/L. Staining of laundry and plumbing may occur at concentrations above 0.3 mg/L. Iron also promotes undesirable bacteria growth (iron bacteria) with a distribution system which results in the deposition of a slimy coating on the piping [2].
Conventional techniques aimed at removing heavy metals usually include chemical precipitation, ion exchange, membrane process, crystallization, and electrochemical treatment [3]. These processes may be ineffective or expensive, especially when the heavy metal ions are contained in solution at concentrations ranging from 1-100 mg/L. Agro-wastes are now being studied for use as adsorbents because of their abundant availability and low cost owing to their relatively high fixed carbon content and the presence of porous structure. Byproducts of agriculture or wood processing are cost-effective and environmentally-benign means to remove heavy metals from water [4].
Although many investigations have been conducted on the ability of some biomass or waste materials to remove Fe(III) from aqueous solutions [5][6][7][8][9][10][11][12][13][14], only a few investigators have studied Fe (II) biosorption. In these studies attention have been focused on carbonaceous materials from biomass and animal parts like crab shell [15], bark of a tree [16], wooden charcoal [17], while few used unmodified biomass waste [18][19]. Recently, studies on the removal of Fe(II) from aqueous solutions have been conducted using synthesized and chemically modified materials. Novel synthetic siliceous waste sorbents with 99.8 % Fe(II) removal efficiency have been studied [20].
Also the use of manganese oxide and iron oxide coated zeolite [21] and modified phosphate dolomite [22] as sorbents have been reported. The application of these materials though with good iron(II) percentage removal would be financially challenging especially in rural areas where this type of contamination is often time prevalent.
This paper studies and compares the iron(II) removal efficiency of maize waste parts namely cob, seed chaff, stalk and sheath in their natural forms. Their removal efficiencies at optimum study conditions would also be considered.

Reagents and solution preparation
All chemicals used were of analytical reagent grade and purchased from Finlab Nigeria Ltd Owerri, Nigeria. A freshly prepared stock solution of ferrous chloride (1000 mg/L) was used as adsorbate and a solution of 50 mg/L were obtained by diluting the stock solution with distilled water.

Collection and preparation of biosorbent
Fully mature maize crops were collected from a farm in Eziobodo in Ihiagwa, Owerri West, Imo State Nigeria. They were washed repeatedly with distilled water to remove dust and soluble impurities. The different parts of the plant which comprised of sheath, stalk, cob and seeds were separated and cut into small pieces. The seeds were crushed and washed through a sieve to remove the starch they contain. These parts were then dried for 5 hours in a conventional oven at a temperature of 100 o C. The dried parts were crushed with a mechanical grinder to a fine powder which was passed through a sieve of 30 mesh and stored in a plastic bottle for further use.

Instrumentation
Characterization of biosorbent was done with Fourier-transform infrared spectrophotometer (FTIR) Agilent Cary 630. Pressed pellets were prepared by grinding the powdered specimens with spectroscopic grade KBr. Characterization was done before and after the biosorption process. The concentrations of Fe(II) in the different solutions were determined using HI 83200 Multiparameter Bench Photometer by HANNA Instruments.

Adsorption studies
Batch adsorption tests were conducted in triplicates to determine the effect of different parameters i.e.; pH, adsorbent dosage and time for maximum removal of Fe(II) from solution by the different parts of maize biomass.

Effect of pH on adsorption
The different parts of maize biomass; cob, sheath, stalk and seed chaff of weight 0.2 g were added to 50 mL of 50 mg/L Fe(II) solutions in glass reactors. Solutions with pH values ranging from 1.0 -4.0 were prepared in triplicates. This pH range was used for the study because the precipitation of metal ions occurred simultaneously at pH higher than 6 [23 -24]. They were covered with aluminum foils and shaken for 120 minutes with a digital shaker. After this time the suspensions were filtered and the concentration of Fe(II) in the filtrates were measured.

Effect of adsorbent dose on adsorption
This study was carried out at the optimum pH obtained for each biomass part. 50 mL of 50 mg/L Fe(II) solutions at optimum pH were transferred in four glass reactors set in triplicates. Adsorbent masses of 0.1 g, 0.2 g, 0.4 g and 0.8 g were then added. They were covered with aluminum foils and shaken for 120 minutes with a digital shaker. After this time the suspensions were filtered and the concentration of Fe(II) in the filtrates were measured.

Effect of contact time
The dependence of metal uptake by biomass adsorbents as a function of time was determined at the optimum pH and adsorbent dose obtained for each biomass part. Five glass reactors containing 50 mL of 50 mg/L Fe(II) solutions at the optimum pH and biomass load for each part were prepared in triplicates. They were covered with aluminum foils and shaken with a digital shaker. A set was removed every 15 minutes and the suspensions were filtered and concentrations of Fe(II) in the filtrate were measured. This process was carried out for a period of 120 minutes.

Evaluation of biosorption efficiency
Concentrations of Fe(II) in the filtrates were determined by diluting each sample to a ratio of 1:50, enabling reading and analysis of the concentration of metal ions. The value of the percentage removal (γ) of metal ions from solution was calculated from the equation [25]: where Co and Ce are the initial and equilibrium concentrations of Fe(II) in the solutions (mg/L).

Results and discussion
Plant biomass or lignocellulose is composed of cellulose, hemicellulose and lignin. Different functional groups have been found on this material by researchers [26][27][28][29][30][31] and are shown in table 1.
FTIR scans of the maize biomass parts used in this study before and after metal ion up-take are shown in Fig. 1a-1d

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The percentage removal of Fe(II) from solution by the different biomass parts as a function of pH is shown in Fig. 2.   Figure 2. Plot of pH vs percentage removal of Fe 2+ by 0.2 g of different biomass parts.
All parts of the biomass studied showed an increase in the up-take of Fe(II) as pH of the solution increased. Metal adsorption by the stalk was uniform and steady for all the studied pH values. The other parts showed a rapid increase in metal uptake after pH of 3. At low pH values (1.0-3.0) more H 3 O + ions will be available to compete with the Fe(II) ions in solution for the adsorption sites of the biosorbents. Also at low pH most of the functional groups on the biomass are protonated [33]. This will reduce the number of binding sites available for the adsorption of Fe(II) ions. The pH can also affect the solution chemistry. In acidic pHs, Fe(II) ions remain with the positive charge, but in alkaline pHs, the hydroxide ions (OH -) cause precipitation of Fe(II) in the form of Fe(OH) 2 leading to decrease in ion concentration in solution not dependent on adsorption.
The effect of biomass load on the percentage removal of Fe(II) from solution by the different biomass parts is shown in Fig. 3. It is plausible that at higher adsorbent dose, there would be more exchangeable sites for the metal ions [34]. This was observed in the seed chaff which showed a steady increase in metal uptake with increase in biomass load, and removed 70 % of the ions from solution at 0.8 g load. However, the adsorption capacity of the other maize biomass parts did not conform to this linearity and showed a consistent alternating increase and decrease in metal removal efficiency as biomass load increased. These observations maybe due to overlap of adsorption sites, arising from overcrowding of adsorbent particles [35]. Also the high adsorbent dose could impose a screen effect of the dense outer layer of the cells, thereby shielding the binding sites from the metal ions [36]. For sheath, stalk and cob best metal removal were achieved at 0.1 g, 0.8 g and 0.4 g respectively. The time for the equilibrium adsorption of the biomass materials for the different adsorbents at pH 4 and optimum biomass load are shown in Fig. 4. Most literature works on the biosorption of Fe(II) from aqueous solutions use carbon, ash, charcoal, minerals or synthetic materials as adsorbents. The percentage removals of Fe(II) in some of these studies under different conditions gave best values as 82 % in 50 minutes at pH 6 [37], 97 % in 90 minutes at pH 5 [38] and 99.8 % in 40 minutes at pH 2 [20]. For all the biomass parts used in this study, the adsorption of Fe(II) increased with increase in contact time. Equilibrium adsorption was reached in approximately 30 minutes for all the parts. The efficiency of the biomass parts in the sorption process were in the order seed, stalk, sheath and cob with values 73 %, 67.1 %, 66.4 % and 66.3 % respectively. The percentage removal of Fe(II) after 30 minutes for all the biomass parts was either constant or decreased as more time was allowed for the process. The decrease in the adsorption quantity after 30 minutes suggests a possible double layer adsorption of the metal ions by weak electrostatic forces between the second layer of ions and the first layer. For the seed however, Fe(II) concentration remained constant after 30 minutes up to 120 minutes, which suggests a possible monolayer adsorption of the ions without intermolecular interactions.

Conclusion
This study has shown that maize biomass can be successfully applied for the removal of Fe(II) ions from aqueous solutions. Though the parts of the plant have similar functional groups, their efficiencies in the removal process differ. The seed chaff which is a waste product in maize starch extraction for domestic and industrial applications is a veritable material for the sorption of Fe(II) from solution with a biosorption efficiency of 73 % in its natural and unmodified form. [2] Department of National health and Welfare Canada, Nutrition recommendations, the report of the Scientific Review Committee, Ottawa, 1990.