Article pubs.acs.org/IECR
Boron Adsorption on Palm Oil Mill Boiler (POMB) Ash Impregnated with Chemical Compounds Hui Jiun Chieng† and Mei Fong Chong†,* †
Department of Chemical and Environmental Engineering, The University of Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor, Malaysia S Supporting Information *
ABSTRACT: Boron removal from synthetic wastewater by using palm oil mill boiler (POMB) ash impregnated with various chemical compounds (citric acid, tartaric acid, salicylic acid, barium chloride, calcium chloride anhydrous, and calcium chloride dihydrate) was being investigated in this study. The characteristics of POMB ash were analyzed based on ash content, carbon content, and BET surface area. The adsorption process was performed in a batch system with various parameters: dosage of POMB ash, residence time, shaking speed, temperature, and pH. The results show that impregnants increased the ability of POMB ash to adsorb boron, and the adsorption efficiency of boron was highly dependent on the pH. The optimization study shows that citric acid was the optimum impregnant on the POMB ash, with maximum boron removal efficiency of 65.69% at dosage of 6 g/50 mL at pH 7.0 with shaking speed of 100 rpm at 25 °C and 12 h of residence time. during recent years.14 Activated carbon is frequently used to purify waters. However, the reduction in boron concentration is negligible using activated carbon. In addition, due to the high cost of activated carbon, many investigators have studied the feasibility of cheap, commercially available materials as its possible replacements.14 Boron removal by adsorption onto ash was studied and proven to be productive.13,15,16 Moreover, ash obtained from the combusted biomass was able to remove 80% of boron from industrial wastewater.15 Palm oil mill boiler (POMB) ash is a byproduct obtained from the burning of biomass in the boiler of a palm oil mill. The ash from the boiler is usually disposed off at no cost at the mill sites.17 Hence, the profuse sources of POMB ash at low cost make it to be an economic choice of adsorbent for boron removal in industrial wastewater treatment. However, its adsorption capacity is low (0.09 mg/g), and it is unlikely to be economically applied to full scale boron containing wastewater treatment.15 Better results can be achieved by pretreating or modifying the adsorbents to enhance their adsorption capacities. One of methods is impregnation of chemical compounds onto the adsorbent. In a study on this topic, boron adsorption has been investigated by using activated carbon impregnated with citric and tartaric acid, and results showed that the amount of boron adsorbed had been doubled.18 Besides, previous researches show that activated carbon impregnated with tartaric acid was the most effective boron adsorbent in both batch and column operation, with adsorption capacities of 2.197 and 3.56 mg/g, respectively.18,19 Not only activated carbon, resin impregnated with citric and tartaric acids have also shown enhancement in boron removal. Ion exchangers of IRA 93, IRA 400 and WOFATIT L 150 were impregnated with citric acid and tartaric
1. INTRODUCTION Boron is extensively used in the manufacturing industries: glass and porcelain, wire drawing, carpets, cosmetics, photographic chemicals, for fireproofing fabrics, and weatherproofing wood.1 Thus, the wide use of boron in industries causes boron contamination to the environment which is mainly through industrial wastewater discharge,2,3 and also by coal combustion, leaching from treated wood, and waste from borate mining and processing.4 In addition, boron is toxic to plants at elevated concentrations. Sensitive plants can tolerate irrigation waters with only 0.3 ppm boron. Excessive intake of boron is harmful to plant growth and causes deleterious effects of yellowish spots on the leaves and plant expiration.5 Moreover, it can cause organisms’ reproductive impediments when the boron concentration exceeds the safe intake level.6 Hence, discharge of boron containing waste to the environment must be controlled at a safe and sustainable level. In Malaysia, there is a legislation requirement by the Department of Environment (DOE) to reduce the boron concentration to below 1 and 4 ppm for Effluent Discharge Standards A and B, respectively.7 Standards A and B are classified corresponding to the location of the industrial area, which are the upstream and downstream regions of the water reservoir for drinking water, respectively. Boron can be removed from wastewaters by different treatment technologies, such as ion exchange,8 precipitation− coagulation,9 membrane filtration,10 adsorption, liquid−liquid extraction,11 and also using aquatic plants.12 Among these technologies, adsorption is the commonly chosen process for boron removal because it is comparatively more useful and economical at low pollutant concentration.13 It has been used for many years as a physical--chemical process, and it is now a major industrial separation technique. Hence, adsorption is suitable to be used for boron removal from wastewater. However, selection of adsorbents is also critical for effective boron removal. The application of low cost and easily available materials in wastewater treatment has been widely investigated © 2013 American Chemical Society
Received: Revised: Accepted: Published: 14658
April 16, 2013 September 7, 2013 September 13, 2013 September 13, 2013 dx.doi.org/10.1021/ie401215n | Ind. Eng. Chem. Res. 2013, 52, 14658−14670
Industrial & Engineering Chemistry Research
Article
sample was placed in a furnace (CWF 1200, Carbolite, U.K.) at the temperature 750 °C for 4 h according to the ASTM D317423 standard for ash content measurement. The ash content was determined as the ratio of final mass to its initial mass as eq 1:
acid to study their boron adsorption capacity. Results showed that citric acid was a more effective impregnant compared to tartaric acid.20 In another study, activated carbon impregnated with salicylic acid has greatly enhanced the boron adsorption from boron solution.21 Both barium and calcium chloride were also studied for impregnation because of their reactivity toward borate ions in water and the possibility of forming complexes between barium or calcium and borate ions.22 Hence, it is possible that impregnation of these chemicals on POMB ash could lead to improvement of the boron sorption characteristics of POMB ash. For the first time this paper presents an analysis of the use of POMB ash impregnated with various chemical compounds of citric acid, tartaric acid, salicylic acid, barium chloride, calcium chloride (anhydrous), and calcium chloride (dihydrate), for boron separation from boron containing solution. The main objective was to determine the optimum impregnant to be impregnated on POMB so that maximum boron adsorption can be obtained. Factors affecting boron adsorption efficiency, such as residence time, temperature, adsorbent dosage, shaking speed, and pH, were investigated. The optimum operating conditions for the POMB ash impregnated with the optimum impregnant identified in this study were also determined.
% of ash content =
final mass × 100% initial mass
(1)
Particles surface area, pore volume, and pore size of ash were obtained by using a BET Surface Area and Porosity Analyzer (ASAP 2020, Micromeritics, USA) through the nitrogen gas adsorption method. Prior to measurement, the samples were degassed at 105 °C for 10 h24 in order to remove moisture and other contaminants. The samples were characterized using a low temperature (77.35 K) nitrogen adsorption isotherm measured over a wide range of relative pressure from 10−6 to 1 atm. The elemental compositions of the organic chemical, including carbon, hydrogen, and nitrogen contents, were obtained by using a CHNS Analyzer (FlashEA 1112, Thermo Fisher Scientific, USA), which uses a combustion process to break down substances into simple compounds, which are then quantified by infrared spectroscopy. 2.3. Impregnation Method. In the present study, the POMB ash was used to be impregnated with various impregnants, as identified in the previous section. 100 g of dry POMB ash was first immersed in 500 mL of impregnating solution containing a predetermined concentration of the impregnant. The resulting impregnated ash was filtered out using a mesh filter and washed using distilled water until the pH became around 7.00, free from the acid or alkaline, before it was dried at 25 °C in shielded open air for 72 h. For impregnation with salicylic acid, the POMB ash filtered was washed using diethyl ether to collect all the unimpregnated salicylic acid. The amount of impregnants impregnated on POMB ash was then determined by the difference between the initial and final concentrations of the impregnation solution using a simple mass balance approach. All the impregnation experiments were duplicated to ensure the consistency of the results. The optimum initial concentration of each impregnant on POMB ash was determined through the investigation of the BET surface area of POMB ash at various impregnants’ concentrations. 2.4. Batch Adsorption Experiment. Batch adsorption tests were carried out to identify the optimum impregnant to be impregnated on POMB ash in order to obtain a novel boron selective adsorbent for boron removal from the industrial wastewater. Synthetic wastewater (50 mL) with a boron concentration of 10 ppm was contacted with impregnated POMB ash in a 250 mL conical flask filled with a predetermined amount ranging from 1 to 5 g. The mixed solutions were shaken at different shaking speeds (75−125 rpm) using a water bath shaker (Water bath WNB 7-45, Memmert, Germany) for a predetermined period of time (2− 24 h) at various temperatures (25−65 °C). The pH was adjusted using dilute hydrochloric acid HCl (96%) (analytical grade, Fisher Scientific, Malaysia) and sodium hydroxide, NaOH (analytical grade, Merck, Germany) solutions. The pH values of the samples were checked from time to time and were adjusted to maintain a constant pH value at all times during the adsorption. After that, the ash was filtered using filter paper (Grade 393, Sartorius Stedim, Germany) and the solutions were analyzed for the contents of boron and impregnants. The
2. EXPERIMENTAL SECTION 2.1. Materials. 2.1.1. Boron Containing Synthetic Wastewater. Ten parts per million of synthetic wastewater was prepared by dissolving a predetermined amount of sodium tetraborate or Na2B4O7·10H2O (industrial grade, Eti Maden Isletmeleri Gn. Md., Turkey) into a predetermined volume of distilled water. As bulk sample will be used in industrial scale at a later stage of study; hence, it will be more feasible and more representable to use distilled water in this study, as the presence of H+ and OH− will have negligible effects. The initial pH of the synthetic wastewater was measured (sensION1 Portable pH Meter, HACH, USA) as 9.0. 2.1.2. POMB Ash. The raw POMB ash used in the present study was obtained from the Seri Ulu Langat Palm Oil Mill, Dengkil, Malaysia. The raw POMB ash with a mixed particle size range was mechanically sieved using a motorized test sieve shaker (T 1, Unit Test, Malaysia) and was categorized by its particle size: smaller than 0.5 mm, 0.5−2.0 mm, and larger than 2.0 mm. A preliminary kinetic test was conducted to investigate the boron uptake by the raw POMB ash of different particle sizes. The experiment was carried out without pH adjustment, and 20 g of ash was used to treat 100 mL of synthetic wastewater with the initial boron concentration of 10 ppm. The samples of different particle sizes were mixed at 100 rpm in conical flasks for 8 h at 25 °C in a water bath shaker (Water bath WNB 7-45, Memmert, Germany). 2.1.3. Impregnants. The selected impregnants to be impregnated on ash were salicylic acid (SA) (analytical grade, Bendosen Laboratory Chemicals, Norway), citric acid monohydrate (CA) (analytical grade, R&M Chemicals, Essex, U.K.), tartaric acid (TA) (analytical grade, R&M Chemicals, Essex, U.K.), calcium chloride anhydrous (CC(an)) (analytical grade, Bendosen Laboratory Chemicals, Norway), calcium chloride dihydrate (CC(di)) (analytical grade, Merck, Germany), and barium chloride dihydrate (BC) (analytical grade, Merck, Germany). 2.2. Ash Characteristics. The characteristics for POMB ash were analyzed to measure ash content, BET surface area, and organic elemental composition. One gram of POMB ash 14659
dx.doi.org/10.1021/ie401215n | Ind. Eng. Chem. Res. 2013, 52, 14658−14670
Industrial & Engineering Chemistry Research
Article
adsorb a significant amount of boron, with almost double the amount of removal by raw virgin POMB ash. This is due to their significantly higher carbon content and higher BET surface area as compared to the other POMB ash particle sizes as shown in Table 1. It is also due to their lower percentage of ash content, which is generally undesirable and is considered an impurity.28,29 Besides, POMB ash with particle size larger than 2.0 mm has similar percentages of ash content and carbon content but lower BET surface area as compared to the range 0.5−2.0 mm. This shows that these two ranges of particle size contain similar particles except that the lower BET surface area of the larger range (>2.0 mm) was attributed to unburned fiber, fruit shell, and crystallized ash and rock, which are slightly larger in size. As a result, virgin POMB ash with particle size of 0.5 and 2.0 mm could effectively adsorb more boron than the other range, and hence, this range of POMB ash was selected for the impregnation and all subsequent studies in this research. Kinetic tests were also performed to understand the effect of particle size and to describe the boron uptake rate on POMB ash. In this study, the kinetic data were analyzed using a pseudo-first-order kinetic model. The pseudo-first-order rate expression given by Lagergren30 and Ho31 is as follows:
experimental runs were duplicated and summarized (Supporting Information, Table S1). The calculated results were then compared, and the impregnated POMB ash with the impregnant which showed the maximum adsorption performance was identified as the optimum impregnant. The impregnated POMB ash with the optimum impregnant was used in the subsequent optimization study to obtain the optimum operating conditions. The optimization study was conducted using a similar experimental procedure of a batch adsorption experiment. All experimental runs were duplicated. The parameters and experimental runs for the optimization study were summarized (Supporting Information, Table S2). 2.5. Analysis Method. The concentration of boron was determined by the carmine method in accordance to APHA Method 8015 using a spectrophotometer (DR 2800, HACH, USA) with boron measuring range from 0.2 to 14.0 ppm. Concentrations of citric and tartaric acids were determined by a acid−base titration method with a strong base of 1 M NaOH.25−27 Salicylic acids were analyzed using a gravimetric method based on the difference between the initial and final weights of the salicylic acid. Concentrations of calcium chloride were analyzed by a calmagite colorimetric method in accordance with APHA Method 8030 using a spectrophotometer (DR 2800, HACH, USA) with a calcium measuring range from 0.05 to 4 ppm calcium as hardness of calcium, CaCO3. Barium chloride concentrations were determined by a turbidimetric method in accordance with APHA Method 8014 using a spectrophotometer (DR 2800, HACH, USA) with barium measuring range from 2 to 100 ppm.
dqt dt
log(qe − qt ) = log(qe) −
ash content (wt%)
carbon content (wt %)
BET surface area (m2/g)
boron removal (%)
raw >2.0 0.5−2.0 2.0 0.5−2.0
