Suja George

Residual aluminium in water defluoridated using activated alumina adsorption – Modeling and simulation studies

The removal of fluoride from drinking water by the method of adsorption on activated alumina is found superior than other defluoridation techniques mostly due to the strong affinity between aluminium and fluoride. Dissolution of aluminium from the alumina surfaces into its free and hydroxide ions in the aqueous medium is reported to be very low, but the presence of high fluoride concentrations may increase its solubility due to the formation of monomeric aluminium fluoride and aluminium hydroxyl fluoride complexes. An Activated Alumina Defluoridation Model Simulator (AAD) has been developed to represent fluoride adsorption on the basis of the surface complexation theory incorporating aspects of aluminium solubility in presence of high fluoride concentrations and pH variations. Model validations were carried out for residual aluminium concentrations in alumina treated water, by conducting a series of batch fluoride adsorption experiments using activated alumina (grade FB101) treating fluoride concentrations of 1-10mg/L, at varying pH conditions. The total residual aluminium in the defluoridated water is due to presence of both dissolved and precipitated Al-F complexed forms. The Freundlich adsorption isotherm was found fit for fluoride adsorption capacity versus residual fluoride concentrations for pH=7.5, and the relationship is given by the linearised equation log(x/m)=logk+(1/n) logC(e) with values of k=0.15mg/g and 1/n=0.45 indicating favorable adsorption. The relationship is linear in the region of low fluoride concentrations, but as concentrations of fluoride increased, the formation of the dissolved AlF(3)(0) complexes was favored than adsorption on alumina, and hence makes the isotherm nonlinear. The AAD simulations can predict for operating fluoride uptake capacity in order to keep the residual aluminium within permissible limits in the alumina treated water.

Synthesis of hydroxyapatite nanorods for application in water defluoridation and optimization of process variables: Advantage of ultrasonication with precipitation method over conventional method

This research work presents the synthesis of hydroxyapatite (Hap) nanorods for defluoridation of drinking water by using both conventional (CM) and ultrasonication with precipitation (USPM) methods. Calcium nitrate was reacted with potassium phosphate in presence of ammonia for controlled pH to synthesize Hap nanorods, which was characterized using FTIR, XRD, SEM, TG-DTA, and TEM/EDS for determining its phase composition, structural and thermal decomposition behavior. When USPM method was used for synthesis, the yield of the Hap nanorods was improved from 83.24±1.0% to 90.2±1.0%, and complete phase transformation occurred with formation of elongated Hap nanorods. Effects of process parameters such as solution pH, contact time and adsorbent dose were studied through response surface methodology (RSM). A simple quadratic model was developed using Central Composite Design (CCD) and optimum parameters for fluoride adsorption process were determined to be pH 7, contact time 3h and adsorbent dose 7g/L for maximum removal capacity. Fluoride removal efficiency was predicted to be 93.64% which was very close to the experimental value obtained at 92.86% using ultrasonically prepared Hap. Fluoride adsorption isotherms fitted the Freundlich isotherm with an adsorption capacity of 1.49mg/g, while the kinetic studies revealed that the process followed pseudo-second order model. The treated water quality parameters such as residual fluoride, calcium leached, total hardness and alkalinity was investigated, and it was observed that all these parameters were within the permissible limits as per WHO and BIS standards.

Hydrodynamic cavitation: an emerging technology for the intensification of various chemical and physical processes in a chemical process industry

Abstract Hydrodynamic cavitation (HC) has been explored by many researchers over the years after the first publication on hydrolysis of fatty oils using HC was published by Pandit and Joshi [Pandit AB, Joshi JB. Hydrolysis of fatty oils: effect of cavitation. Chem Eng Sci 1993; 48: 3440–3442]. Before this publication, most of the studies related to cavitation in hydraulic system were concentrated to avoid the generation of cavities/cavitating conditions. The fundamental concept was to harness the energy released by cavities in a positive way for various chemical and mechanical processes. In HC, cavitation is generated by a combination of flow constriction and pressure-velocity conditions, which are monitored in such a way that cavitating conditions will be reached in a flowing system and thus generate hot spots. It allows the entire process to operate at otherwise ambient conditions of temperature and pressure while generating the cavitating conditions locally. In this review paper, we have explained in detail various cavitating devices and the effect of geometrical and operating parameters that affect the cavitation conditions. The optimization of different cavitating devices is discussed, and some strategies have been suggested for designing these devices for different applications. Also, various applications of HC such as wastewater treatment, preparation of nanoemulsions, biodiesel synthesis, water disinfection, and nanoparticle synthesis were discussed in detail.

Modeling and Simulation studies for Aluminium - Fluoride Interactions in Nalgonda Defluoridation Process

A model simulator NALD-2 has been developed to study the interactions of fluoride and aluminium in the Nalgonda Defluoridation Process, which principally involves the preferential adsorption of fluoride ions onto insoluble aluminium hydroxides that are formed from alum hydrolysis reactions and undergo precipitation. This model represents the defluoridation mechanism taking into account the charged behavior of the amphoteric aluminium hydroxide colloids, charged site densities, and fluoride complexation reactions in order to predict the extent of defluoridation and concentrations of dissolved and colloidal aluminium in the treated water as functions of pH, alum dosage and raw fluoride concentrations. Model validations were carried out using secondary data of Selvapathy & Arjunan (1995) for total residual aluminium in Nalgonda treated water. Slight variations in pH into the acidic range cause substantial increase in colloidal aluminium and hence pH and alum dosages are important control parameters to limit residual aluminium in treated water. The NALD-2 simulator helps in predicting optimum alum dosages for minimum residual aluminium in the treated water. It was concluded that the dosages of alum recommended in the Nalgonda method cannot bring down the residual Al below the permissible limit of 0.2mg/L in the treated water.

Treatment of textile dyeing industry effluent using hydrodynamic cavitation in combination with advanced oxidation reagents

Treatment of textile dyeing industry (TDI) effluent was investigated using hydrodynamic cavitation (HC) and in combination with advanced oxidation reagents such as air, oxygen, ozone and Fentons reagent. Slit venturi was used as the cavitating device in HC reactor. The effects of process parameters such as inlet pressure, cavitation number, effluent concentration, ozone and oxygen flow rate, loading of H2O2 and Fentons reagent on the extent of reduction of TOC, COD and color were studied. Efficiency of the hybrid treatment processes were evaluated on the basis of their synergetic coefficient. It was observed that almost 17% TOC, 12% COD, and 25% color removal was obtained using HC alone at inlet pressure of 5bar and pH of 6.8. The rate of reduction of TOC and COD decreased with dilution of the samples. HC in combination with Fentons reagent (FeSO4·7H2O:H2O2 as 1:5) was most effective with reduction of 48%TOC and 38% COD in 15min and 120min respectively with almost complete decolorization (98%) of the TDI effluent. Whereas HC in combination with oxygen (2L/min) and ozone (3g/h) produced reduction of 48% TOC, 33% COD, 62% decolorization and 48% TOC, 23% COD, 88%, decolorization of TDI effluent respectively.

In-vitro synthesis of marble apatite as a novel adsorbent for removal of fluoride ions from ground water: An ultrasonic approach

Marble waste powder consisting of calcium and magnesium compounds was used to synthesize a novel biocompatible product, marble apatite (MA) primarily hydroxyapatite (Hap) for applications in defluoridation of drinking water. Synthesis of marble apatite was carried out by using calcium compounds (mixture of hydroxide and nitrate) extracted from marble waste powder which was treated with potassium dihydrogen phosphate at 80°C under alkaline conditions using conventional precipitation method (CM) and ultrasonication method (USM). Qualitative analysis of synthesized marble apatite from both the methods was carried out using FTIR, phase analysis by XRD and microstructure analysis by SEM and TEM. When ultrasonication (USM) method was used, the yield of marble apatite was improved from 67.5% to 78.4%, with reduction in crystallite size (58.46nm), lesser agglomeration and comparatively well-defined spherical morphology compared to the CM method. Studies also include estimation of the defluoridation capacity of MA as an adsorbent for drinking water treatment and effects of process parameters such as pH, contact time, initial fluoride concentration, dosage and presence of other co-ions on fluoride removal capacity. The results showed that the experimental adsorption capacity of the marble apatite synthesized using USM method was significantly higher (1.826mg/g) than marble apatite synthesized using conventional method (0.96mg/g) at pH 7 with a contact time of 90min. The mechanism of adsorption was studied, and it was observed that Langmuir isotherm model fitted best to the experimental data, while the kinetic studies revealed that the process followed pseudo-second order model. This novel compound, marble apatite synthesized from marble waste powder is found to be promising for defluoridation of drinking water and will help in alleviating the problems of fluorosis as well as reduce the problems of disposal of marble waste.

An advanced pretreatment strategy involving hydrodynamic and acoustic cavitation along with alum coagulation for the mineralization and biodegradability enhancement of tannery waste effluent

In the present study, coagulation followed by cavitation was studied as a pretreatment tool for tannery waste effluent (TWE) with the aim of reducing its COD, TOC, TSS etc. and enhancing its biodegradability to make it suitable for anaerobic digestion. Initially, coagulation was applied to TWE using alum as a coagulant. The residual pH of treated effluent was found to be around pH of 4.5 where maximum COD and TSS reduction was achieved. In order to enhance the efficiency of pretreatment process, coagulated tannery waste effluent (CTWE) was further subjected to hydrodynamic cavitation (HC) and ultrasonication (US). In case of HC, effect of process parameters such as inlet pressure and dilution on the treatment of CTWE was initially investigated. Lower operating pressure (5 bar) was more favorable for the treatment of CTWE using HC in order to enhance the biodegradability index (BI) from 0.14 to 0.57 in 120 min. The CTWE samples when subjected to 50% dilution, HC pretreatment exhibited higher percentage and quantum reduction in TOC and COD. On the other hand, pretreatment of TWE using coagulation followed by US demonstrated that BI of effluent was enhanced from 0.10 to 0.41 in 150 min. Energy efficiency evaluation for all processes at their optimized conditions was done based on the actual amount of COD reduced per unit energy delivered to the system. Coagulation followed by HC for the pretreatment of TWE was found to be six times more energy efficient as compared to coagulation followed by US.

Application of hydroxyapatite and its modified forms as adsorbents for water defluoridation: an insight into process synthesis

Abstract Fluorosis is a major scourge in many countries caused by prolonged consumption of drinking water with high fluoride content found in groundwater resources. Hydroxyapatite (Hap) and its composite forms are excellent biomaterials that recently gained attention as efficient adsorbents, owing to its physical and chemical nature as it can substitute both cationic and anionic complexes present in an aqueous solution in its atomic arrangement. Its biological nature, biocompatibility and biodegradability along with its chemical characteristics such as crystallinity, stability, ion adsorption capability and highly specific catalytic activity make it suitable for a variety of applications especially in water treatment for fluoride removal. This review describes various techniques for synthesis of a wide variety of biogenic, synthetic, composite and modified forms of Hap for application in water defluoridation. Hap derived from natural sources or synthesized using conventional methods, hydrothermal, sol-gel or advanced sonication-cum-precipitation technique varied in terms of its crystallinity, structure, size, etc., which affect the fluoride removal capacity. The advantage and disadvantages of various synthesis methods, process parameters and product characteristics have been compiled, which may help to identify a suitable synthesis method for a desired Hap product for potential application and future perspectives in water treatment.

Process modeling for estimation of colloidal aluminium in alum treated water during defluoridation

Fluoride contamination is present in most groundwater resources and aluminium (Al) based techniques are mostly adopted for defluoridation of water because of the high affinity between aluminium and fluorides. The presence of residual aluminium in treated drinking water has become a cause of worldwide concern and aluminium based coagulants have come under scrutiny with findings of neuro-toxicity when aluminium is ingested in different forms. This paper presents the theoretical model NALD-2 developed to study for interactions of fluoride and aluminium in the alum based Nalgonda Defluoridation Process and that can predict for extent of defluoridation as well as concentrations of dissolved and colloidal aluminium in the treated water with respect to changes in pH, alum dosage and initial fluoride concentrations. This model represents the defluoridation mechanism taking into account the charged behavior of the amphoteric aluminium hydroxide colloids, available charged site densities and fluoride complexation reactions. Colloidal aluminium concentration is a function of alum dosage and pH of the water. The NALD-2 model helps in predicting alum dosages for minimum residual aluminium in alum treated water, as the fluoride removal is dependent on the available charged sites on the aluminium hydroxide particles