presence of toxic and stable compounds in the environment has led to major health and environmental problems . phenol is one of the most common contaminants found in wastewater of industries such as resin , plastics , explosives , steel , leather , etc . the phenol released from these industries causes contamination of the environment , in particular water sources . this compound forms compounds such as chlorophenol which are classified as priority pollutants due to their certain features such as high toxicity , carcinogenicity , impact on the taste and odor , accumulative , low biodegradability potential , and many other adverse effects on human health and other living things , and has attracted the attention of many environment experts ( 1 , 4 , 5 ) . the health effects caused by exposure to phenol can be determined by the amount of the phenol adsorbed as well as the contact time . these effects could vary from mucosal and skin irritation and burns to systemic toxicity as well as decreased blood pressure , increased heart rate and coma ( 6 ) . studies have shown that consumption of 8 to 15 mg of phenol could result in death ( 7 ) . phenol will quickly enter the circulatory system of the livings due to its high permeability , regardless of the manner of exposure ( skin contact or mucosal adsorption ) ( 8) . the presence of phenol in drinking water treated with chlorine leads to formation of side products and makes the water non - potable and harmful by giving it an unpleasant taste and odor ( 9 ) . in addition , considering the carcinogenicity potential of phenol to humans and other living things , remarkable concerns are raised even in low concentrations ( 10 ) . toxic effects of phenol include penetration into the cells and cytoplasm , clotting and damaging the crucial cells in the body ( 1013 ) . thus , detection and determination of the concentration of phenolic compounds in wastewater and their subsequent removal in all steps of production , treatment and evacuation of wastewater into the sources appears to be necessary . regulations of world health organization limits the phenol concentration to below 0.1 g l ( ppb ) and according to the regulations of moef ( ministry of environment and forests ) its maximum allowed amount in the output of the industries and surface water is considered 1 mg l ( 10 , 14 ) . when releasing wastewater into the surface water and for agricultural and drinking uses , the maximum allowed amount of phenol concentration is determined as 1 mg l and 0.5 g l by environmental protection agency standard and standard institute of iran , respectively ( 15 , 16 ) . various processes are used for removal of phenolic compounds from aqueous media , most common of which include extraction , adsorption with activated carbon , steam distillation , electrochemical methods , radiation , advanced oxidation , chemical oxidation , adsorption , and biological treatment . the majority of these methods suffer from disadvantages such as high cost of treatment , the need for additional filtration , production of dangerous side products , low efficiency and applicability for a limited range of contaminant concentrations ( 17 , 18 ) . the adsorption process is one of the most common methods applied widely for removal of organic and inorganic contaminants from aqueous solution ( 19 ) . the adsorption process is one of the simplest , most efficient , and cost effective alternatives for removal of phenol from aqueous media which is capable of removing most forms of organic material ( 1923 ) . the low costs and prevalence of the utilized adsorbents such as agriculture waste has led this method to be widely investigated and applied for phenol removal . activated carbon , clay , polymer materials , sludge , zeolite , chitosan and chitin are some of the absorbents used for phenol removal ( 2431 ) . commercial activated carbon demonstrates high efficiency for removal of phenol , however , considering high costs of the preparation and reduction of commercial activated carbon , it is not cost effective to use . this has compelled researchers to try to develop novel cost effective adsorbents ( 32 ) . hence , in this study , the application of pomegranate peel ash is investigated as an adsorbent for removal of phenol from aqueous solutions . all chemicals used in this study were purchased from merc and aldrich companies . dried peel of pomegranate was supplied from ferdows city . to obtain carbon from these peels , they were then dried in an oven at 105 c for sixty minutes . in the next step , they were ground as finely as possible and placed in an oven at 450 c for sixty minutes to obtain the carbon . it was then sieved using standard astm sieves with the size in the range of 0.150.85 mm . finally , the samples were cooled and kept in a desiccator . to determine the optimum parameters for any kind of adsorbent , the experiments were performed on samples of water contaminated with phenol with concentrations of 10 to 100 mg l , which were prepared from stock solution of 1000 mg l of phenol , and initial ph values of 2 to 12 by addition of 0.1 and 1 normal sulfuric acid and sodium hydroxide . the monitoring took place by a denver ultra basic - ub10 ph meter made in usa . following addition of the adsorbent to the solutions in erlenmeyer flasks , the mixture was stirred at constant temperature of 301 c at stirring rate of 150 rpm using a stirrer equipped with incubator , model ks 4000i control , made by ika company of germany . this process was repeated twice . finally , the sampling of phenol concentration took place using a t80 uv / vis spectrometer in 15 to 210 minutes after separation of the adsorbent by centrifuging at 4000 rpm for 10 minutes , and the calibration curve was plotted at wavelength 505 nm and evaluated on the basis of the equation [ phenol ] = 9.11abs505 - 0.756 , ( r 0.99 ) ( 8 , 10 ) . in the next step , the langmuir and freundlich adsorption isotherms were studied . the following equations were used to determine the adsorption capacity and efficiency ( 9 ) : qe = ( cice)/w ; adsorption ( % ) = ( cic)/ci where : qe = adsorption capacity ( mg / g ) , ci= initial concentration of the adsorbed material in the solution ( mg / l ) , ce= equilibrium concentration of the adsorbed material after equilibrium takes place ( mg / l ) , v= volume of liquid inside the reactor ( l ) , w= adsorbent mass ( g ) . figure 1 illustrates the impact of initial ph of the solution on phenol adsorption efficiency by pomegranate peel ash . as shown in figure 1 , the maximum phenol removal is obtained as 93.72% which occurs at ph=7 . as observed in figure 2 , the phenol removal efficiency increase from 69.7 to 97.06% with the increase of the adsorbent dose from 0.1 to 0.7 g / l . however , the adsorbed phenol rate decreases with respect to each gram of the adsorbent ( adsorption capacity ) . furthermore , once the adsorbent dose reaches above 0.6 thus , the optimum dose of the adsorbent was chosen as 0.6 g / l and used in the next steps of the experiments . the acquired results from the investigations demonstrate that , the adsorption capacity and phenol removal efficiency increases considerably at initial stages of the adsorption and after approximately 120 minutes reaches the maximum amount ( 95.96% ) . therefore , the retention time of 120 minutes was chosen as the optimum time in this study ( figure 3 ) . as can be seen in figure 4 , the adsorption capacity of the adsorbent , increases with the increase of phenol concentration and reaches 148.38 mg / g at initial concentration of 100 mg from 16.43 mg / g at 10 mg . in contrast , the efficiency of phenol removal at the same time period demonstrates a reverse trend . the isotherm models turn experimental results into applicable formulas with useful parameters and make them easy to use in designing processes . study of the isotherms may help interpret the reactions between the adsorbed material and the adsorbent . in other words , isotherm shows the relationship between concentration of the phenol in solution and the adsorbed phenol rate by solid phase when the two phases are in the equilibrium state . langmuir and freundlich equations are widely used in description of adsorption isotherms in solid / liquid systems . in langmuir this model is used to estimate the maximum rate of uptake , the amount of which is obtained in the experiments . however , freundlich isotherm is based on the adsorption on heterogeneous surfaces and unlike the langmuir model , does not determine the maximum capacity of the adsorbent . the isotherms are shown for both models in figures 5 and 6 based on the experimental data and obtained parameters from non - linear regression . figure 1 illustrates the impact of initial ph of the solution on phenol adsorption efficiency by pomegranate peel ash . as shown in figure 1 , the maximum phenol removal is obtained as 93.72% which occurs at ph=7 . as observed in figure 2 , the phenol removal efficiency increase from 69.7 to 97.06% with the increase of the adsorbent dose from 0.1 to 0.7 g / l . however , the adsorbed phenol rate decreases with respect to each gram of the adsorbent ( adsorption capacity ) . the acquired results from the investigations demonstrate that , the adsorption capacity and phenol removal efficiency increases considerably at initial stages of the adsorption and after approximately 120 minutes reaches the maximum amount ( 95.96% ) . therefore , the retention time of 120 minutes was chosen as the optimum time in this study ( figure 3 ) . as can be seen in figure 4 , the adsorption capacity of the adsorbent , increases with the increase of phenol concentration and reaches 148.38 mg / g at initial concentration of 100 mg from 16.43 mg / g at 10 mg . in contrast , the efficiency of phenol removal at the same time period demonstrates a reverse trend . the isotherm models turn experimental results into applicable formulas with useful parameters and make them easy to use in designing processes . study of the isotherms may help interpret the reactions between the adsorbed material and the adsorbent . in other words , isotherm shows the relationship between concentration of the phenol in solution and the adsorbed phenol rate by solid phase when the two phases are in the equilibrium state . langmuir and freundlich equations are widely used in description of adsorption isotherms in solid / liquid systems . in langmuir theory this model is used to estimate the maximum rate of uptake , the amount of which is obtained in the experiments . however , freundlich isotherm is based on the adsorption on heterogeneous surfaces and unlike the langmuir model , does not determine the maximum capacity of the adsorbent . the isotherms are shown for both models in figures 5 and 6 based on the experimental data and obtained parameters from non - linear regression . the ph of a solution plays an important role in adsorption process , especially in adsorption capacity . this results from the amount of surface charge of the adsorbent , amount of ionization of the material present in the solution as well as separation of the functional groups present in the adsorbing sites of the adsorbent ( 34 ) . thus , the adsorption of phenol was studied as a function of hydrogen ion concentration , to determine the effect of the initial ph variation of the samples on the phenol adsorption process by pomegranate peel ash . the initial ph values of the samples were adjusted within 212 in each series of the experiments . the utilized adsorbent was 0.04 g in 100 ml of the solution under experiment and the initial concentration of phenol was 50 mg l in all samples . as inferred from figure 1 , by increasing the ph value from 2 to 7 , the phenol removal efficiency also increases to 93.72% from initial value of 71.4% and decreases the adsorption on polar adsorbents . then , following increasing the ph value above 7 , efficiency of the process decreases , as in ph=12 the removal efficiency is obtained as 57.96% . in general , at higher ph values , adsorption decreases ( ph>7 ) since phenol is in the form of a salt which loses its negative charge easily , and hence becomes difficult to adsorb . in addition , in this condition , the presence of oh ions inhibit phenol ions from being adsorbed ( 35 ) . the adsorbent dose is one of the most important parameters which determine the adsorbent capacity for determination of phenol concentration . therefore , the adsorption experiments were performed using an adsorbent dose in the range of 0.10.7 g in 100 ml solution containing 50 mg l at optimum ph=7 for sixty minutes at 232 c , to determine the effect of adsorbent dose on the adsorption process by pomegranate peel ash . as observed in figure 2 , the efficiency of phenol removal has gradually increased up to 97.06% with a gentle slope , following increasing the adsorbent up to 0.7 g / l . in other words , there are still sites on the adsorbent which are not saturated during the adsorption process . considering the fact that , the concentration of phenol is kept constant in all experiments , increasing the amount of the adsorbent has led to enhancing the ratio of the adsorbent to the adsorbed phenol , which provides more surface and sites for adsorption . furthermore , the results show that , when adsorbent dose reaches above 0.6 mg / l , a remarkable enhance is not observed in phenol adsorption by the adsorbent . it seems , after increasing the adsorbent dose up to a certain amount , the maximum adsorption rate is reached and thus , from this amount on , the remaining amount of the contaminant will stay constant even after addition of more adsorbent ( 36 ) . although phenol removal increased by increasing the adsorbent dose , the adsorbed phenol rate per gram of adsorbent decreased , because concentration of dissolved phenol ( 50 mg / l ) decreased by being adsorbed on unsaturated active sites of the adsorbent . on the other hand , the increase of the adsorbent results in agglomeration of the particles and their adherence to each other ( 37 ) . g / l of the adsorbent is chosen as the optimum dose of the adsorbent for the rest of the study . the contact time is one of the most important parameters for determination of the equilibrium state of adsorption process . the impact of contact time on the adsorption process was studied at 23  2 c , with initial phenol concentration of 50 mg / l and an adsorbent dose of 0.6 g / l in time ranges between 15210 minutes . figure 3 depicts the impact of contact time on the adsorption capacity and efficiency of phenol removal by pomegranate peel ash . the adsorption capacity and efficiency of phenol removal increases at first stages of the adsorption remarkably , and then reaches the maximum amount ( 95.96% ) after 120 minutes . the adsorption capacity of pomegranate peel ash is also obtained as 79.97 mg / g . therefore , the retention time of 120 minutes is chosen as the equilibrium time . in general , removal of the adsorbed material is fast at first stages of the process , but then decreases gradually until reaching the equilibrium state . this phenomenon may be interpreted as ; at the first stages of the adsorption process , there are many available sites on the surface of the adsorbent and over time , the unoccupied sites on the surface of the adsorbent are hardly occupied due to depletion forces between molecules of the adsorbent and the solution phase . on the other hand , the efficiency of phenol removal is high at first stages , because of the high availability of the sites on the surface of the adsorbent for adsorbing the contaminants . once the sites on the surface of the adsorbent are saturated upon adsorption process , the adsorption rate is controlled by adsorption and desorption of the contaminants on the adsorbent ( 36 ) . similar results have been reported by gengiz and cavaz ( 37 ) , and senthilkumar et al . effect of initial concentration of phenol on the adsorption efficiency was studied at 23  2 c with the adsorption dose of 0.6 g / l for 120 minutes . as shown in figure 4 , the adsorption capacity of the adsorbent increases by increasing phenol concentration , whereas the efficiency of phenol removal demonstrates a reverse trend . when initial concentration of phenol increases to 100 ml from 10 ml , the adsorption capacity increases to 148.38 mg g from 16.43 mg g. this may be explained as ; with the increase of phenol concentration , the mass transfer driving force and hence the speed of phenol molecules passing through solution to the liquid layer surrounding the adsorbent and finally toward surface of the adsorbent , increases ( 40 , 41 ) . on the other hand , the efficiency of phenol removal ( removal efficiency ) decreases from 98.06% to 89.03 mg / l by increasing initial concentration of phenol from 10 mg this could be attributed to the constant sites for adsorption and increasing the number of molecules of the adsorbed material . in addition , similar results are obtained for phenol adsorption from aqueous solutions by clay and other low price adsorbents ( 4248 ) . studying the isotherms may help interpret how reactions between the adsorbent and the adsorbed material take place . in other words , an isotherm demonstrates the relationship between phenol concentration of the solution and amount of the phenol adsorbed by solid phase when the two phases are in equilibrium state . the freundlich isotherm is an experimental relationship based on adsorption on heterogeneous surfaces and unlike the langmuir model , does not specify the maximum adsorption capacity and thus , is applicable only at medium or low concentrations ( 4951 ) . the isotherms are shown in figures 5 and 6 for both models on the basis of experimental data and parameters resulted from nonlinear regression . the correlation coefficient of the langmuir model ( r=0.8674 ) is lower than that of the freundlich model ( r=0.9056 ) which indicates the fact that the freundlich model is more suitable for interpretation of equilibrium adsorption of phenol on pomegranate peel ash . on the other hand , once the parameter 1/n value is in between 0.11 , it shows appropriate adherence to the freundlich isotherm and indicates that the adsorption is pleasant as was in this study . the ph of a solution plays an important role in adsorption process , especially in adsorption capacity . this results from the amount of surface charge of the adsorbent , amount of ionization of the material present in the solution as well as separation of the functional groups present in the adsorbing sites of the adsorbent ( 34 ) . thus , the adsorption of phenol was studied as a function of hydrogen ion concentration , to determine the effect of the initial ph variation of the samples on the phenol adsorption process by pomegranate peel ash . the initial ph values of the samples were adjusted within 212 in each series of the experiments . the utilized adsorbent was 0.04 g in 100 ml of the solution under experiment and the initial concentration of phenol was 50 mg l in all samples . as inferred from figure 1 , by increasing the ph value from 2 to 7 , the phenol removal efficiency also increases to 93.72% from initial value of 71.4% and decreases the adsorption on polar adsorbents . then , following increasing the ph value above 7 , efficiency of the process decreases , as in ph=12 the removal efficiency is obtained as 57.96% . in general , at higher ph values , adsorption decreases ( ph>7 ) since phenol is in the form of a salt which loses its negative charge easily , and hence becomes difficult to adsorb . in addition , in this condition , the presence of oh ions inhibit phenol ions from being adsorbed ( 35 ) . the adsorbent dose is one of the most important parameters which determine the adsorbent capacity for determination of phenol concentration . therefore , the adsorption experiments were performed using an adsorbent dose in the range of 0.10.7 g in 100 ml solution containing 50 mg l at optimum ph=7 for sixty minutes at 232 c , to determine the effect of adsorbent dose on the adsorption process by pomegranate peel ash . as observed in figure 2 , the efficiency of phenol removal has gradually increased up to 97.06% with a gentle slope , following increasing the adsorbent up to 0.7 g / l . in other words , there are still sites on the adsorbent which are not saturated during the adsorption process . considering the fact that , the concentration of phenol is kept constant in all experiments , increasing the amount of the adsorbent has led to enhancing the ratio of the adsorbent to the adsorbed phenol , which provides more surface and sites for adsorption . furthermore , the results show that , when adsorbent dose reaches above 0.6 mg / l , a remarkable enhance is not observed in phenol adsorption by the adsorbent . it seems , after increasing the adsorbent dose up to a certain amount , the maximum adsorption rate is reached and thus , from this amount on , the remaining amount of the contaminant will stay constant even after addition of more adsorbent ( 36 ) . although phenol removal increased by increasing the adsorbent dose , the adsorbed phenol rate per gram of adsorbent decreased , because concentration of dissolved phenol ( 50 mg / l ) decreased by being adsorbed on unsaturated active sites of the adsorbent . on the other hand , the increase of the adsorbent results in agglomeration of the particles and their adherence to each other ( 37 ) g / l of the adsorbent is chosen as the optimum dose of the adsorbent for the rest of the study . the contact time is one of the most important parameters for determination of the equilibrium state of adsorption process . the impact of contact time on the adsorption process was studied at 23  2 c , with initial phenol concentration of 50 mg / l and an adsorbent dose of 0.6 g / l in time ranges between 15210 minutes . figure 3 depicts the impact of contact time on the adsorption capacity and efficiency of phenol removal by pomegranate peel ash . the adsorption capacity and efficiency of phenol removal increases at first stages of the adsorption remarkably , and then reaches the maximum amount ( 95.96% ) after 120 minutes . the adsorption capacity of pomegranate peel ash is also obtained as 79.97 mg / g . therefore , the retention time of 120 minutes is chosen as the equilibrium time . in general , removal of the adsorbed material is fast at first stages of the process , but then decreases gradually until reaching the equilibrium state . this phenomenon may be interpreted as ; at the first stages of the adsorption process , there are many available sites on the surface of the adsorbent and over time , the unoccupied sites on the surface of the adsorbent are hardly occupied due to depletion forces between molecules of the adsorbent and the solution phase . on the other hand , the efficiency of phenol removal is high at first stages , because of the high availability of the sites on the surface of the adsorbent for adsorbing the contaminants . once the sites on the surface of the adsorbent are saturated upon adsorption process , the adsorption rate is controlled by adsorption and desorption of the contaminants on the adsorbent ( 36 ) . similar results have been reported by gengiz and cavaz ( 37 ) , and senthilkumar et al . effect of initial concentration of phenol on the adsorption efficiency was studied at 23  2 c with the adsorption dose of 0.6 g / l for 120 minutes . as shown in figure 4 , the adsorption capacity of the adsorbent increases by increasing phenol concentration , whereas the efficiency of phenol removal demonstrates a reverse trend . when initial concentration of phenol increases to 100 ml from 10 ml , the adsorption capacity increases to 148.38 mg g from 16.43 mg g. this may be explained as ; with the increase of phenol concentration , the mass transfer driving force and hence the speed of phenol molecules passing through solution to the liquid layer surrounding the adsorbent and finally toward surface of the adsorbent , increases ( 40 , 41 ) . on the other hand , the efficiency of phenol removal ( removal efficiency ) decreases from 98.06% to 89.03 mg / l by increasing initial concentration of phenol from 10 mg this could be attributed to the constant sites for adsorption and increasing the number of molecules of the adsorbed material . in addition , similar results are obtained for phenol adsorption from aqueous solutions by clay and other low price adsorbents ( 4248 ) . studying the isotherms may help interpret how reactions between the adsorbent and the adsorbed material take place . in other words , an isotherm demonstrates the relationship between phenol concentration of the solution and amount of the phenol adsorbed by solid phase when the two phases are in equilibrium state . the freundlich isotherm is an experimental relationship based on adsorption on heterogeneous surfaces and unlike the langmuir model , does not specify the maximum adsorption capacity and thus , is applicable only at medium or low concentrations ( 4951 ) . the isotherms are shown in figures 5 and 6 for both models on the basis of experimental data and parameters resulted from nonlinear regression . the correlation coefficient of the langmuir model ( r=0.8674 ) is lower than that of the freundlich model ( r=0.9056 ) which indicates the fact that the freundlich model is more suitable for interpretation of equilibrium adsorption of phenol on pomegranate peel ash . on the other hand , once the parameter 1/n value is in between 0.11 , it shows appropriate adherence to the freundlich isotherm and indicates that the adsorption is pleasant as was in this study . the acquired results demonstrated that pomegranate peel ash shows good capability for removal of phenol from aqueous solutions . the adsorption rate will increase by increasing the amount of the adsorbent due to contact surface increase . at higher concentrations of phenol , a decrease in adsorption rate was observed . the adsorption capacity obtained as 148.38 mg / g when applying 0.6 g pomegranate peel ash for removal of phenol from 100 ml of solution containing 100 mg of phenol in 120 minutes at ph=7 . furthermore , it was observed that , pomegranate peel ash is capable of removing more than 98% of the phenol from aqueous solutions with initial concentrations in between 10100 mg / l . on the other hand , the obtained experimental results in this study were evaluated by two well - known models , freundlich and langmuir , and the coefficient values acquired indicate that the freundlich model is more appropriate compared to the langmuir model .
introductionphenol and its derivatives are the most common poisonous compounds which are stable in aqueous media and lead to many health issues . in this study , 
 application of the carbon resulted from pomegranate peel is investigated in removal of phenol by adsorption method.methodsto perform this cross - sectional study , first , samples of phenol with concentrations of 10 to 100 mg / l were prepared for six months in 2016 . 
 then , the impacts of parameters such as ph , adsorbent dosage , contact time , and initial concentration of phenol in adsorption process were investigated independently in gonabad chemistry lab using a spectrophotometer at 505 nm as the wavelength . 
 furthermore , adherence of the samples to the isotherm models of langmuir and freundlich was determined by excel 2016 and descriptive statistical methods were then reported.resultsthe obtained results demonstrated a maximum adsorption capacity ( ash ) of 148.38 mgg-1 at ph 7 , initial concentration of 100 mg l1 , and temperature of 23  2 c . 
 the phenol removal rate was found to correlate directly to the adsorbent dosage and contact time , and inversely to the initial concentration of phenol . 
 in addition , the investigations showed that the adsorption of phenol on the pomegranate peel ash follows the freundlich model well with a correlation coefficient of r2 0.9056.conclusionpomegranate peel ash could be used as an efficient and low - cost adsorbent for phenol removal from aqueous media .