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ISSN : 1225-7672(Print)
ISSN : 2287-822X(Online)
Journal of the Korean Society of Water and Wastewater Vol.29 No.5 pp.565-573
DOI : https://doi.org/10.11001/jksww.2015.29.5.565

# Comparable Influencing Factors to evaluate the Phosphate Removal on the Batch and the fix-bed Column by Converter Slag

Sang-Ho Lee
Department of Environmental Engineering, Sangmyung University
Corresponding author : Sang Ho Lee leesh@smu.ac.kr
July 17, 2015 September 24, 2015 October 7, 2015

## Abstract

The influencing factors to remove phosphate were evaluated by converter slag (CS). Experiments were performed by batch tests using different CS sizes and column test. Solutions were prepared at the different pH and concentrations. The maximum removal efficiency was obtained over 98% with the finest particle size, CSa within 2 hours in batch tests. The removal efficiency was increased in the order of decreasing size with same amount of CS for any pH of solutions. The adsorption data were well fitted to Freundlich isotherm. From the column experiment, the specific factors were revealed that the breakthrough removal capacity (BRC) xb/mcs, was decreased by increasing the influent concentration. The breakthrough time, tb was lasted shorter as increasing the influent concentration. The pH drop simultaneously led to lower BRC drop during the experimental hours. The relation between the breakthrough time and the BRC to influent concentration was shown in the logarithmic decrease. Results suggested that the large surface area of CS possessed a great potential for adsorptive phosphate removal. Consequently particle size and initial concentration played the major influencing factors in phosphate removal by converter slag.

# 회분식과 연속흐름 칼럼에서 전로슬래그에 의한 인제거 영향에 미치는 요소에 관한 연구

이 상호
상명대학교 환경공학과

## 1.Introduction

Phosphorus is a very important element for growth of organisms in most ecosystems. Efforts to reduce phosphorus concentrations in aqua systems have mainly concerned with improving the water quality in water body. Once released in exceed concentration into the water body, excessive amounts of phosphate supply to water bodies may cause eutrophication and subsequent deterioration of water quality. Even the trace amount of phosphate in municipal wastewater and industrial wastewater is often responsible for algae blooming in stagnant water bodies. Removal technologies such as various biological removal, constructed wet lands, chemical precipitation using coagulant, adsorption by activated carbon and fly ashes have been employed to remove effectively from wastewater systems. Of those technologies some biological treatments and wet land systems cannot be applied for the stringent guidelines.

The objective of this research was to evaluate the significance of influencing factors of converter slag for removing phosphate. In batch experiments, the maximum phosphate adsorption kinetics, solution pH, different sizes, initial phosphate concentration were evaluated. Subsequently, three columns with the different initial concentration were conducted to investigate the breakthrough time and breakthrough adsorption capacity whether those are fitted to arithmetic or logarithmic relation.

## 2.Materials and methods

### 2.1.Materials

Waste converter slag (CS) used was a by-product derived from iron ore industry, Pohang city Korea. The converter slag was examined by X-ray diffraction spectroscopy. The result of analysis by XRD was illustrated in the previous reference(Lee and Lee, 2007). The chemical compositions are composed as follows; FeO 23.86%, CaCO3 38.68%, SiO2 12.71%, MgO 11.15%, MnO 2.17%, Al2O3 2.308%, others are less than 1.0%. The slag grain were available in four different sizes, 0.075-0.15 mm (CSa), 0.15-0.425mm (CSb), 0.425-2mm (CSc) and 2-4.75mm (CSd), respectively. Slag samples were washed with distilled water and dried at 105°C for 24 hours before every experiment. The phosphate stock solution containing 1,000 mg/L was prepared by anhydrous KH2PO4 in distilled water.

### 2.2.Batch experiments for phosphate adsorption

Batch experiments were employed to verify the effect of changing pH of solution, adsorption capacity and size effect during experimental hours. The phosphate adsorption capacity was examined using two batch equilibrium techniques as described below. The first adsorption experiments were performed at 3mg/L to understand the removal kinetics for the grain size effect of different weight and to find out adsorption kinetics. Various masses (1, 3, 5, 7, 10, 25, 50 g) were used for CSa and CSb. Several masses (10, 25, 75, 100, 150 g) were used for CSc and CSd. The second adsorption experiments were performed to verify the effect of pH solution and the intensity of solution concentration using 100g of CSd with several pH (7, 7.5, 8, 8.5) in 250mL glass Erlenmeyer flasks containing 100mL of different phosphate solution (1, 3, 5, 10mg/L). All converter slag in Erlenmeyer flask was shaken in a constant temperature shaker (SI-900R, JEIO Tech, Korea) at 20°C with 120rpm. The measurement of changing pH was employed using a pH meter (Orion 4STAR, Thermo) and phosphate was analysed by DR4000 Spectrophotometer at the given experimental intervals for every experiment.

### 2.3.Continuous column experiment

Column studies were conducted using three HDPE tubes of 7.5cm ID and 36cm of packed height of 45cm height. The column was filled by 2,780g of CSd with 53% porosity. The solution was fed through the bottom of vertical column by a peristaltic pump (Masterflex Variable-speed Digital Drive, Cole-Parmer) with a head (L/S Easy-load II, Cole-Parmer). The flow rate was 40mL/min, 21min of retention time, 1.026m/h of line velocity and 2.85/h of specific velocity. Each tube was fed 10, 30 and 50 mg/L of influent concentration, respectively. The effluent samples were intermittently collected at time intervals until the column reached a concentration greater than 95% of the feed concentration.

## 3.Results and discussion

### 3.1.Effect of particle size and weight

The removal efficiency was more than 80% in 2 hours with CSa, 8 hours with CSb. The final removal efficiency was 94% for CSa, 90% for CSb, 81% for CSc and 73% for CSd, respectively. It was increased as the surface area increase as the particle size was fine. Those were not exactly proportional to the particle size. The amount of phosphate adsorbed increased with the adsorption time for all experiments. The kinetic constants for the different weight of every size of converter slag were calculated to elucidate the adsorption rate using the formula of the first-order reaction as shown in Table 1. The removal rates increased as decreasing size and increasing weight at a given experimental hour.

The increasing values of removal kinetic appeared to correspond to the order of increasing the weight of converter slag. It must be pointed out that the removal kinetic constants were correlated to the increasing weight of the given any size of converter slag. The contribution of removal kinetic appears to be relatively highly relevant and dependant on particle size which means smaller size contributed the larger adsorption surface area. However the relation between size and weight for every size of converter slag is not exactly correlated. The rate constants for same amount (10, 25, 50g) of CSa and CSb were much higher than those of CSc and CSd. It was much influenced by increasing converter slag weight 10g to 50g for CSa and CSb as small size comparing with CSc and CSd. Also the removal rate can be compared that the rate kinetic increased twice higher at 50 g than 1g of CSa and CSb. However the rate kinetic did not change much for any weight of CSc and CSd. Comparing the rate kinetic for small size and big size, the rate of small size revealed four times higher than that of small size. However it was not different appearance though increasing converter slag weight 10g to 50g for CSc and CSd as large size. It was increased as increasing for low weight of converter slag such as 1g to far up 50g of CSa and CSb. The increasing weight contributed to increase the rate kinetic for CDa and CDb as small size. It was not much increased even though the weight of converter slag increased 10g to 150g for CSc and CSd as large size. Therefore it was found that the removal rate was highly affected by adsorbent size, on the other hand increasing weight did not much contribute the kinetic rate for CDc and CDd as bigger size. The smaller the particle size led the higher final pH of solution. It seems that surface area increased as small particle. The removal rate efficiencies of phosphate removal from aqueous solution were found to increase as increase of surface area of particle.

The equilibrium data is important to analyse fitting classic adsorption isotherm equation. The Langmuir and Freundlich adsorption isotherm models were utilized to describe the equilibrium data. The Langmuir isotherm model is based on that the phosphate adsorption occurs on a solid surface by monolayer adsorption, which is expressed following equation (1).

$q e = b q m C e 1 + b C e$
(1)

Where, qe is the amount of phosphate adsorbed at equilibrium (mg/g), Ce is phosphate concentration in equilibrium (mg/L), qm is maximum phosphate adsorption capacity on converter slag (mg/g), and b is a constant related to phosphate on converter slag (L/g). The Freundlich equation is represented by following equation (2).

$q e = K F C e 1 / n$
(2)

Where qe is the amount of phosphate adsorbed on the solid phase (mg/g), Ce is the equilibrium phosphate concentration in solution phase (mg/L), KF is an indicator of the adsorption capacity and 1/n is the heterogeneity factor which has a lower value for more heterogeneous surfaces. Application of the Freundlich equation to the data is illustrated graphically in Figure 1. Adsorption constants for Freundlich isotherm were illustrated in Table 2. However, result of Langmuir isotherm was negligible. Linearized forms of Freundlich isotherm for the phosphate adsorption on converter slag were shown in Figure 1.

The KF and 1/n values for the Freundlich isotherm and regression coefficients by equation (2) were shown in Table 2. The correlation coefficients ranging were from 0.92 to 0.99. The constant n for n>1 indicates favourable adsorption. Higher adsorption capacity of CS was likely related to the higher surface area of CS. It can be postulated that the removal kinetics were greater at high concentration with bigger size of CS and greater at lower concentration with smaller size of CS.

### 3.3.Removal characteristics by column experiment

The characteristics of breakthrough curves strongly depended on operating concentration. The effect of phosphate concentration was explored at 10, 30 and 50 mg/L at constant flow rate as described in the experimental conditions. The volume of solution treated before breakthrough point was considerably reduced by increasing the influent concentration as shown in Figure 3 which implied breakthrough time was reached faster at low influent concentration. The initial pH of solutions were prepared at pH 7, the pH drop was appeared slowly for 10 mg/L as shown in Figure (2A) in long time, however it was appeared fast drop in a short time for 50 mg/L as shown in Figure (2C). For the results, the sharp drop of pH led to commonly the high efficiency of phosphate removal on high influent concentrations. The corresponding results appeared to the order of increasing of phosphate influent concentration. The increase of influent phosphate concentration led to shorter breakthrough time and sharp drop of pH of effluent.

The final solution pH reached at 9.7 for 10mg/L, 9.2 for 30mg/L and 8.6 for 50mg/L with the initial pH of 7.0. The sharp decrease in pH at the initial run time for 50mg/L however gradual decrease of pH was revealed as experimental time for 10mg/L. The removal trend does not exactly controversial proportion to the pH drop of solution throughout run time shown in Figure 2. It seems that the slow decrease of removal induced gradual drop of pH at 10mg/L of phosphate solution, however faster pH drop with higher concentration at 50mg/L was appeared than any other concentration.

The breakthrough time was decreased as increasing the influent concentration increasing from 10 to 50mg/L. The breakthrough adsorption capacity was calculated by the Equation (3) during the continuous flow column experiment (Crittenden, et al., 1987a).

$x m b = x b m cs = Q C o − C b 2 t b m cs$
(3)

Where, (x/m)b is breakthrough adsorption capacity (g/g), mcs is weight of converter slag(g), Q is flow rate (m3/day), C0 is influent concentration of phosphate(g/m3), Cb is concentration of phosphate at breakthrough point(g/m3), tb is breakthrough time(day). The flow rate, Q and the weight of converter slag, mcs was fixed. Therefore the breakthrough adsorption capacity mainly depended upon breakthrough time, tb for this study. The results indicated that adsorption capacity decreased 3.54g P/kg adsorbent to 1.72g P/kg adsorbent by increasing the influent concentration from 10 to 50mg/L as shown in Figure 3.

Logarithmic number of adsorption capacity and logarithmic number of breakthrough time was revealed by the linear decrease as increasing the influent concentration as shown in Figure 4. The adsorption breakthrough time obtained by changing influent phosphate concentration from 10 to 50 mg/L with conditions described procedure section. A decrease in phosphate concentration led to a later breakthrough time 432, 99 and 42 hours as increasing concentration, respectively. The results were revealed that the relation between concentration and breakthrough adsorption capacity was linearly logarithmic relation.

## 4.Conclusions

The by-product converter slag has been applied successfully for the removal of phosphate from aqueous solution. The volume of solution treated before breakthrough time was considerably reduced by increasing the influent phosphate concentration. converter slag exhibited significant phosphate adsorption capacities. The influencing factors for the removal of phosphate by converter slag were depending on particle size, time, initial concentration and pH of solution. In batch tests, particle size and initial concentration were the main influencing factors. The maximum phosphate adsorption earned by the application of Freundlich isotherm was mainly affected by its size. In column tests, the logarithmic conversions of both the breakthrough time and breakthrough adsorption capacity were fitted linear relation with influent concentration and decreased as increasing influent concentration.

## Figure

Freundlich isotherm for phosphate adsorption on converter slag CSa, CSb(1A) and CSc, CSd(1B).

Breakthrough curves expressed as concentration profiles (C/Co) and pH versus time interval for the influent concentration of 10 mg/L (2A), 30 mg/L (2B) and 50 mg/L (2C).

Relation between logarithmic number of breakthrough time (hour) and logarithmic number of adsorption capacity (mg P/g CS) versus different influent concentrations.

## Table

Removal kinetic (hr-1) for different size and varying weights of converter slag.

The KF and 1/n values for the Freundlich isotherm and regression coefficients of equation

Breakthrough time and breakthrough adsorption capacity for the different influent concentrations by column experiments

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