Removal of Nitrate from Aqueous Solution by Adsorption with Amberlite IR 400 and Duolite A 378 Ion Exchange Resins
Abstract
Nitrate contamination of water sources is becoming one of the most important water quality concerns in Iraq. The maximum contaminant level (MCL) for nitrate is 45 mg/L. In this paper two types of resins (strong-base Amberlite IR 400 and weak-base Duolite A 378) were used to remove nitrates from water in some variables including (amount of resin, pH, agitation rate, time, and the concentration of nitrates) were tested. Langmuir and Freundlich adsorption isotherm were tested on resins to study their behaviors. It was concluded that strong and the weak resins are approximately the same in the removal of nitrates with better efficiency of the removal. The strong resin on the other hand showed a better ability in removal than weak resin.
Introduction
ater is the basis of life on earth and within the industry, agriculture and the activities of human beings as waste carriers of many heavy metals and sediments which are concentrated in groundwater or rivers.
This paper investigates the nitrate ion concentration in water and the initial concentration ratio of solution was chosen according to the surface and groundwater hydrochemical study in Alhishama zone in Badra – Jassan basin at eastern of Iraq, that ranging from (82.6-117 mg/l) [1], this high level has been impacted by certain agricultural, commercial, or industrial activities and as it known that high ratio of nitrates poses a serious humanitarian risk such as cancer diseases of the digestive system and blue baby syndrome in Infants under six months [2] .
There are many ways for water treatment; ion exchange by resin was used in this paper with two types of resins (strong-base Amberlite IR 400 and wake -base Duolite A 378) which resins are considered the most promising owing to their chemical stability and ability to control surface chemistry. The characteristics of adsorption behavior are generally inferred in terms of both sorption kinetics and equilibrium isotherms.
The objectives of this work are: 1) Measuring the efficiency of two types of resins to remove nitrates; 2) to obtain experimental equilibrium data; 3) to determine the suitable model describing isotherm.
Mass transfer at ion exchange in stirred batch system
When an ion exchange particle is brought in contact with a solution, there is a static liquid film formed around it, depending on the rate of flow of liquid past the particle [3]. Nitrate ions diffuse from the solution through the film into the beads and Cl- ions diffuse out of the beads crossing the film into the solution, figure (1).
Figure (1): Mass transfer at ion exchange
The mass balance of the batch system is represented by the following equation [4]:
('q^-)/'t= m/v ('C_b)/'t (1)
– The adsorption capacity at time t, qt (mg /g) was calculated as follows:
q_e=(C_0-C_e)v/m (2)
Where C'' and Ce (mg/ l) are the liquid phase concentrations of solutes in the initial and given time t, v (l) is the solution volume and m (g) is the mass resin [5,6].
– Langmuir model considers taking place on the surface of metal ions by adsorption monolayer homogenous. The Langmuir adsorption isotherm equation can be expressed as [7]:
1/(x'm)=1/(k c_e v_m )+1/v_m (3)
– Freundlich adsorption isotherm equations can be expressed as
x/m='k (ce)'^(1/n) (4)
Taking logarithms on both sides in the equation
log x/m=logk+1/n logce (5)
Where x = amount of dye ion adsorbed (mg), m = weight of adsorbent used (mg),
ce= equilibrium dye concentration and K, 1/n = constant characteristics of the system.
The value of K = adsorption capacity and 1/n = sorption intensity can be calculated from the graph. From the straight line, we can calculate the slope (1/n) and intercept (log k) [8].
Materials and methods
Resins pretreatment
A strong base ion exchange resin as shown in table (1) are used in this study [9].
Table (1): Characteristics of the resins
Amberlite IR 400 Duolite A 378
Resin type Strong Base Anion Weakly Basic Anion
Ionic form Cl- Cl-
Particle size 0.60-0.75 mm 0.3-1.2 mm
Theoretical capacity ' 1.40 meq/ml 1.4 mmol/ml
Function groups Quaternary Ammonium Tertiary amine
Resins were pretreated by washing resins with deionized water to remove possible organic and inorganic impurities sticking to the surface, soaking the resin for 3 hrs in 2 M hydrochloric acid (HCl) 36.5 %, density 1.185 g/ml prepared by using 169.49 ml of HCl then diluted to 1000 ml with distilled water. Then washing the resin with distilled water to remove excess of the diluents and soaking the resins in 2 M sodium hydroxide (NaOH) for 3 hours to convert Cl- to OH form, NaOH prepared by dissolving 80 g of NaOH in 1000 ml distilled water. After that rinse resins by distilled water for many times until the excess of Na+, H+ groups were removed from the resin. Repeating until the pH of the solution reaches ' 8.1 and storing in a closed container after drying at 70 ''C for 2 hrs and desiccating.
Batch Experiments
The adsorption of nitrate on resin was studied by the batch technique. The general procedure was used for this study is described as follows:
– An initial nitrate concentration of (70, 90,100 and120 mg/l) prepared by dissolving (0.815 g) of KNO3 in 1000 ml deionized water as a stock solution and diluted daily according to dilution law:
V1 C1 = V2 C2 (6)
– The equilibrium time was investigated for both Amberlite IR400 and Duolite A378; then it was kept constant for all following experiments at 190 min.
– Effect of pH on metal removal was studied by carrying out the study at different (5, 6, 7 and 8) pHs. Sodium hydroxide and hydrochloric acid are used to adjust pH of solutions. Samples were analyzed by the UV spectrophotometer at (215 nm) wavelength.
– The effect of the agitation rate was studied by adding 0.5 g of resin into 100 ml of a solution of 70 mg/l concentration at optimum pH determined previously. The solution was agitated using a water bath shaker at different stirring rates (0,100, 200,250,500 rpm) under a constant temperature of 30''C.
– The initial concentration of nitrate remains constant along the experiments (70 mg/l), except when studied varying of initial concentration that (70, 90,100,120) mg/l.
– The effect of metal solutions was studied by varying range of resin amount that (0.05, 0.1, 0.3, 0.5and 1) gram for both Amberlite IR400 and Duolite A378 into 100 ml of nitrate solution of 70 mg/L concentration at optimum pH and agitation rate determined previously.
Results and discussion
Effect of pH
The results showed that the amount of adsorbed nitrate increases at low pH (pH= 5 for IR 400 and pH= 6 for A378). This behavior may Interprets as proton excess in solution at lower pH that increases the number of positive charges on the adsorbed surfaces. Results show that IR 400 resin is more active than A378 resin removal of nitrates as shown in table (2) and figure (2)
The amount of metal ion adsorbed was calculated as:
Effect of Initial concentration
The percentage of nitrate removal was found to increase with increased nitrate concentrations that indicate two types of experimental resins are effective and suitable in removal nitrate at high concentrations. Moreover, the weak A378 anion resin efficiency is an approach to strong IR400 anion resin in nitrate removing at the same conditions as shown in table (3) and figure (3).
Table (3): Effect of Initial concentration on Nitrate Removal
Effect of resin amount
The amount of resin has been studied because it is a very important parameter in the selection of the amount to remove nitrate. The results showed when increasing the amount of resin, the percentage of removal was increased, this attributed increasing to the dose of adsorbent that could provide a greater surface area or ion exchange sites added to the initial concentration. At low resin amounts IR400 strong base anion exchange resins operating in the chloride ion have been successfully used for this service more than A378 weak base anion resin. The results show as table (4) and figure (4) below:
Effect of agitation rate:
It was noted that the nitrate adsorption rate increased with increasing agitation rate that reach to 250 rpm, which shows the correct transmission of ions from solution to adsorbent binding sites. At high agitation rate, the efficiency of removal was decreased, which is attributable to the attractive force of interionic becomes stronger at high electrolyte solution, especially for weak A378 resin as shown in table (5) and figure (5).
Analysis of Adsorption isotherm
Adsorption isotherm results presented in the table (6) and figures (6, 7, 8 and 9) for Amberlite IR400 and Duolite A378 resins respectively.
In this study, Freundlich and Langmuir were studied to their importance to establish the most appropriate correlation for the equilibrium curves to compare the effectiveness of different adsorbent materials under different operational conditions.
The results show that resin isotherm near to Langmuir model for both types of resins and strong resin IR400 provide higher chemical interaction with nitrate as shown by (k) values.
Conclusions
' The present study shows that strong anion Amberlite IR400 resin and weak anion Duolite A378 resin were effective for the adsorption of nitrate ions from aqueous solution (preferably strong Amberlite IR400resin), that mean chloride ionic form is suitable for nitrate ion exchange.
' The effects of process parameters like pH, metal ion concentration, adsorbent concentration and adsorbent size on process equilibrium were studied.
' IR 400 resin with best pH=5, is more active than A378 resin with best pH= 6 in removal of nitrates, where the amount of adsorbed nitrate increases at lower pH, which attributed to increase the number of positive charges on the adsorbed surfaces.
' Both Amberlite and Duolite A378 resins are suitable in removal nitrate at high concentrations.
' The removal percentage increased with the resin amount increasing, this provides a greater surface area or ion-exchange sites for affixed initial solute concentration.
' The nitrate adsorption rate increased with increasing agitation rate that reach to 250 rpm, but at high agitation rate, the efficiency of removal was decreased, which is attributable to the attractive force of interionic becomes stronger at high electrolyte solution.
' The resin isotherm fit Langmuir model for both types of resins.