Abstract
Fluoride pollution in groundwater is a major concern in rural areas. The flower petal of Shorea robusta, commonly known as sal tree, is used in the present study both in its native form and Ca-impregnated activated form to eradicate excess fluoride from simulated wastewater. Response surface methodology (RSM) was used for experimental designing and analyzing optimum condition for carbonization vis-à-vis calcium impregnation for preparation of adsorbent. During carbonization, temperature, time and weight ratio of calcium chloride to sal flower petal (SFP) have been considered as input factors and percentage removal of fluoride as response. Optimum condition for carbonization has been obtained as temperature, 500 °C; time, 1 h and weight ratio, 2.5 and the sample prepared has been termed as calcium-impregnated carbonized sal flower petal (CCSFP). Optimum condition as analyzed by one-factor-at-a-time (OFAT) method is initial fluoride concentration, 2.91 mg/L; pH 3 and adsorbent dose, 4 g/L. CCSFP shows maximum removal of 98.5% at this condition. RSM has also been used for finding out optimum condition for defluoridation considering initial concentration, pH and adsorbent dose as input parameters. The optimum condition as analyzed by RSM is: initial concentration, 5 mg/L; pH 3.5 and adsorbent dose, 2 g/L. Kinetic and equilibrium data follow Ho pseudo-second-order kinetic model and Freundlich isotherm model, respectively. Adsorption capacity of CCSFP has been found to be 5.465 mg/g. At optimized condition, CCSFP has been found to remove fluoride (80.4%) efficiently from groundwater collected from Bankura district in West Bengal, a fluoride-contaminated province in India.
Highlights
Fluoride has dual effect on living system
Optimum condition for carbonization has been obtained as temperature, 500 °C; time, 1 h and weight ratio, 2.5 and the sample prepared has been termed as calcium-impregnated carbonized sal flower petal (CCSFP)
Viz. weight ratio of calcium chloride to SFP, temperature and time were varied according to the design as specified by the Design Expert Software 8.0.6 and percentage removal of fluoride by the calcium-impregnated adsorbent samples prepared at different operating conditions, were considered as the response or output variable
Summary
Fluoride has dual effect on living system. While fluoride concentration greater than 1.5 mg/L leads to skeletal and dental fluorosis, concentration lower than 0.5 mg/L is responsible for tooth-decay since low level of fluoride helps to strengthen the enamel (Razbe et al 2013). The techniques used for removal of fluoride may be broadly divided into four categories, i.e., membrane-separation (Meenakshi and Maheshwari 2006), ion-exchange (Mohapatra et al 2009), precipitation-coagulation Ion-exchange can remove fluoride up to 95% but this technique is very costly due to high cost of resin and post-treatment at low pH (Mohapatra et al 2009). A number of adsorbents can be used to defluoridate water (Karthikeyan and Llango 2007; Zhang et al 2011); due to good reactivity of calcium (Ca2?) and fluoride (F-), researches are trending towards the impregnation of calcium on the adsorbent for enhancement of removal of fluoride from its contaminated water (Bhaumik et al 2012; HernandezMontoya et al 2012; Zhang et al 2011)
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