Environmental Pollution and Public Health Special ...

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Special Track within iCBBE: ... treatment by the steel slag column, the residual phosphorus ... mixture of steel slag and quartz sand at a volume ratio of 7׃3 or.
Environmental Pollution and Public Health Special Track within iCBBE: (EPPH 2010) http://www.icbbe.org/epph2010/

June 21-23, 2010 Chengdu, China

Conference Program Guide Sponsors: -

IEEE Engineering in Medicine and Biology Society, USA Sichuan University, China Wuhan University, China Centre for Advanced Water Technology, Singapore

Adsorption removal of phosphorus from aqueous solution by steel slag columns Hua ZHANG1,2, Xuehong ZHANG1,2, Shaoyuan BAI2, Yinian ZHU2,*, Yuzhou GONG2 1. Institute of Light industry and Food Engineering, Guangxi University, Nanning 530004, China; 2. College of Environmental Science and Engineering, Guilin University of Technology, Guilin 541004, China * e-mail address: [email protected] Abstract—Removal of phosphorus from aqueous solution by steel slag column and the effects of the addition of boiler slag, furnace ash and quartz sand were investigated. After the adsorption treatment by the steel slag column, the residual phosphorus concentration was less than 0.5 mg/L. The adsorption performance of the steel slag column could be enhanced by the addition of quartz sand. The columns that were filled with the mixture of steel slag and quartz sand at a volume ratio of 7‫׃‬3 or with the mixture of steel slag and furnace ash at a volume ratio of 7‫׃‬3 indicated the longest run-life and breakthrough time of 26 or 24 days, respectively. Furthermore, the mixing of boiler slag, furnace ash or quartz sand into the steel slag column could effectively mitigate the clogging and hardening risks. Keywords- steel slag; boiler slag; furnace ash; quartz sand; column adsorption; phosphorus removal

I.

INTRODUCTION

Phosphorus is one of the essential limiting factors for the eutrophication of surface waters, while the phosphorus removal efficiency of conventional biological wastewater treatment plants is often low due to mechanical, financial, and limited personnel reasons. Phosphorus exists in municipal wastewaters in different forms, including total phosphorus, soluble phosphorus, and particulate phosphorus. In general, primary and secondary treatment is effective in removing particulate phosphorus. Phosphorus in the secondary effluents is mostly soluble and is present as orthophosphate [1]. However, only orthophosphate is consumed for phytoplankton growth causing eutrophication in surface water bodies. Therefore, an efficient removal of this constituent from the secondary effluent of municipal wastewater treatment plants has been of growing interest. Adsorption has been considered as an alternative approach for P remove since the adsorption process can be operated simply, produces little sludge, and enables P recovery. Considerable attention has been paid to the development of effective and low-cost adsorbents and to evaluate their capacity for phosphorus removal. If inexpensively alternative adsorbents can be developed, it would be beneficial to the environment and have attractive commercial value. In this respect, various materials such as natural sands, furnace slag, steel slag, zeolite, diatomite, activated oyster shell, and red mud have been investigated for P adsorption [2,3]. When adsorbents contact with water, P will be exchanged with the water on the interface until a dynamic equilibrium is

reached. Adsorbents and water concentrations are stabilized when adsorbent-to-water and water-to-adsorbent rates become equal. Adsorption-desorption and precipitation-dissolution may be involved in these processes [4]. The equilibrium situation is usually described by a Langmuir adsorption isotherm, and which is used to calculate the maximum P adsorption capacity of adsorbents. According that, the furnace slag adsorption capacity is 44.2 g/kg. Although zeolite is usually used for ammonium ion removal from wastewater, it still has a P adsorption capacity of 2.15 g/kg [5]. The previous study had demonstrated that pH, chemical composition, duration, contact time and temperature are the important factors in adsorption process. The pH is an important factor for adsorption mechanism. Adsorption is dominant at pH value below 6, while chemical precipitation is dominant when pH value is above 8. The chemical composition of the substrates has a great impact on the adsorption efficiency of phosphorus. Adsorbents usually contain certain amounts of alumina (Al2O3), iron, magnesium and calcium oxides, which may significantly contribute to phosphorous removal by adsorption and precipitation processes [6-8]. Slag particles contain a significant amount of soluble metal ions such as magnesium and calcium. Those metal ions are responsible for chemical precipitation and complexation. Oyster shells contain high content of calcium (approximately 96% as CaCO3) and P adsorption capacity of 16 g/kg. The phosphate is precipitated in the presence of Ca2+ and high pH as a key mechanism [9,10]. Phosphorus adsorption was found to be endothermic; thus an increase in temperature could favor phosphorus adsorption [11]. The presence of high levels of Ca, Al and Fe oxides in industrial by-products steel slag suggests that this material can absorb P, and the use of this industrial by-product can own positive environmental benefits, which can significantly reduce the need for raw materials and the energy use and emissions produced during the mining, processing and transportation of those materials. Therefore, the objective of this paper was to investigate the effectiveness of steel slag and to evaluate whether the addition of materials, including furnace ash, boiler slag and quartz sand would contribute to phosphorus removal from water with column adsorption experiments, all of these minerals have been reported to have high potential to removal P from wastewaters. Furthermore, the influences of adsorbent mixing ratio and contact time on phosphorus removal were also investigated.

Bioinformatics and Biomedical Engineering (iCBBE) 2010, 4th International Conference on Environmental Pollution and Public Health (EPPH2010), Digital Object Identifier: 10.1109/ICBBE.2010.5517077. Publication Year: 2010, Pages: 1-4.

II.

MATERIALS AND METHODS

A. Materials Steel slag was collected from Guilin Mechanical Manufacturing Mill. The furnace ash was taken from Guilin Capacitor Mill. While the Boiler slag and quartz sand were obtained from the canteen and campus of Guilin University of Technology. All samples were washed with distilled water and then dried at 105–110oC for 12h. X-ray fluorescence was used to determine the principal chemical components of the adsorbents. The steel slag consists of calcium silicates and ferrites which was mineralogically mixed with oxides of iron, calcium and magnesium (Ca(Mg0.93Fe0.07)SiO4: 83%, (Co0.6Zn0.4)Cr2O4: 14%, Ce4As3: 2%, CdIn2Te4: 1%). Furnace ash consists essentially of silicates and aluminosilicates of calcium and other bases (CaCO3: 8%, SiO2: 63%, Al(Al0.69Si1.22O4.85): 27%, F3.5Yb0.5Zr0.5: 2%). The major chemical composition of boiler slag was SiO2 (69%), Al(Al0.83Si1.08O4.85) (30%) and TiO2 (1%). The principal chemical components of quartz sand are SiO2 (73.7%), Al2O3 (73.7%), Fe2O3 (8.75%) and (Ca,Mg)O (2.15%). The effluent of municipal sewage treatment plant was employed as the influent of the continuous adsorption experiments, and the concentration was within the range of 1.0 mg/L-1.5 mg/L. B. Continuous adsorption experiments Several plastic columns of 8cm inner diameter and 100cm height were employed in this study. The SS Column was filled with the steel slag, while the SB Column, the SF Column and the SQ Column were filled with the steel slag - boiler slag mixture (SB), the steel slag - furnace ash mixture (SF) and the steel slag - quartz sand mixture (SQ) at the volume ratio of 7:3, respectively. For evaluation the effect of adsorbent mixing ratio and contact time on phosphorus removal, the P adsorption performances of adsorbents mixture at various volume ratios (6:4, 7:3, 8:2) and various contact times (1h, 1.5h, 2h, 2.5h) were investigated subsequently. In continuous adsorption experiments, the secondary effluent from a municipal sewage treatment plant was percolated through the columns at a steady velocity 0.4 m/h by using a peristaltic pump. The mean hydraulic residence time of the solution in the column was 2h. The distilled water was run through the columns for 24h prior to starting the experiments in order to wet the columns and to establish equilibrium between the adsorbent and water. Effluent samples were collected and monitored at one-day intervals. For each column experiment, an entire effluent concentration history curve for total phosphorus was recorded. Phosphorus concentration was analyzed by the molybdenum blue method. All glassware and sample bottles were soaked in diluted HCl solution for 12h, and then washed and rinsed three times with ultrapure water. Ultrapure water was used for the preparation of solutions. All experiments were conducted in duplicate and the average values were used for data analysis.

III.

RESULTS AND DISCUSSION

A. Column experiments Column experiments were conducted to examine the feasibility of the absorbents for phosphate removal and to find the concentration breakthrough pattern. With the initial phosphorus concentration of 1.42 mg/L, the adsorption experiments were operated until the concentration of phosphorus in the effluent had reached 0.5 mg/L. The experimental results indicated that the effluent P concentrations in all columns were almost constant and very low for the first 6 days, and then started to show a decrease in performance. The residual phosphorus concentrations increased with the adsorption time, from 0.5 mg/L (Fig.1). It was probably due to the larger surface area of the adsorbents being available at the beginning of the operation, and there were many sorption sites for P. The breakthrough time of the SS Column (steel slag only) was the shortest one among the four columns. The effluent phosphorus concentration of the SS Column increased from 0.05 mg/L at the initial period to 0.53 mg/L at day 19, and reached the breakthrough point. During the operation period, white suspended solids appeared in the effluent from the fifth day and increased following the operation times, and the white suspended solid was the precipitation of P by Ca as a precipitate of hydroxyapatite, which was a prevailing mechanism responsible for the P removal. At the end of the operation period, 2 days before the phosphate concentration reaches 0.5 mg/L, the hardening and clogging phenomenon appeared. The steel slag in the SS Column was taken out and washed with ultrapure water, dried at 105–110oC for 12h. Subsequently, it was packed into the column once again and the adsorption test was repeated. The mean phosphorus concentration of the influent was 1.42 mg/L and the adsorption performance was satisfied (