Synthesis and physicochemical characterization of Zn/Al chloride layered double hydroxide and evaluation of its nitrate removal efficiency

doi:10.1016/j.desal.2010.02.003

Desalination

Volume 256, Issues 1–3, June 2010, Pages 120-128

https://doi.org/10.1016/j.desal.2010.02.003 Get rights and content

Abstract

Deleterious effects of nitrate on health are well known. A laboratory study was conducted to investigate the ability of Zn–Al–Cl layered double hydroxide for the removal of nitrate from synthetic nitrate solution. In the present study Zn–Al–Cl LDH was synthesized by co-precipitation method and was characterized using SEM, XRD, FTIR and TGA–DSC. To know the practical applicability, a detailed nitrate removal study was carried out. The removal of nitrate was 85.5% under neutral conditions, using 0.3 g of LDH in 100 mL of nitrate solution having initial concentration of 10 mg/L. Adsorption kinetic study revealed that the adsorption process followed first order kinetics. Adsorption data were fitted to linearly transformed Langmuir isotherm with R² (correlation coefficient) >0.99. Thermodynamic parameters were also calculated to study the effect of temperature on the removal process. In order to understand the adsorption type, equilibrium data were tested with Dubinin–Radushkevich isotherm. The percentage removal was found to decrease gradually with increase in pH and the optimum pH was found to be 6. The presence of competitive anions reduced the nitrate adsorption in the order of carbonate > phosphate > chloride > sulphate. The Zn–Al–Cl LDH exhibited low desorption and poor regeneration.

Introduction

Nitrate–nitrogen (NO₃–N) concentration in surface and groundwater has increased in many locations in the world. In recent times, the extensive use of chemical fertilizers and improper treatment of wastewater from the industrial sites and urban sites has led to several environmental problems. Rural areas characterized by heavy agricultural activities are the most susceptible locations to groundwater NO₃–N contamination. One of the agricultural activities contributing to the NO₃–N contamination is livestock. The other problem is the over application of nitrogen based fertilizers. This is the largest source and the primary concern of NO₃–N contamination in groundwater. Several nitrogenous compounds, including ammonia, nitrite and nitrate have been frequently present in drinking water and various types of agricultural, domestic and industrial wastewater [1], [2]. US Environmental Protection Agency (EPA) has set the maximum contamination level as 10 mg/L of NO₃⁻–N [3], [4]. Deleterious effects of nitrate on health are well known. Elevated nitrate concentrations in drinking water sources present a potential risk to public health. It can stimulate eutrophication which causes water pollution due to heavy algal growth. Acute poisoning occurs within 30 min to 4 h after ingestion of plants or water high in nitrates. Thus, the problem occurs very quickly and often the cattle are observed to be normal one day and dead the next day [3]. A very early sign is salivation followed by frequent urination. Soon after, the cattle exhibit difficult breathing, increased respiratory rate, and dark brown or “chocolate” colored blood and mucous membranes. The animals then become weak, reluctant to move, and have convulsions before they die. If pregnant cattle receive a dose that is not quite deadly, they may abort soon after recovering. Nitrate contaminated water supplies have also been linked to outbreaks of infectious diseases in humans [2]. Excess nitrate in drinking water may cause methemoglobinaemia also called a blue baby disease, in newborn infants [5]. Literature survey revealed that nitrate ion also causes diabetes [6] and is a precursor of carcinogen.

Background nitrate concentrations in surface waters are usually below 5 ppm, and higher concentrations are often observed in groundwater. Recent surveys revealed that the nitrate levels have been increased in drinking water supplies in the European Community, the United States, Canada, India, etc. Nitrate is considered to be relatively non-toxic to adults, concentrations greater than 50 ppm can be fatal to infants under six months of age. Increased nitrate concentrations in groundwater have caused the shutdown of wells and rendered aquifers unusable as water source. Surface waters also have experienced seasonal nitrate violations. As a result there is renewed interest in the removal of nitrates from raw water. Unfortunately the policy of countermeasures, especially concerning agriculture and environment to limit pollution by nitrates, is efficient only in the long term. So, technical solutions become obligatory. A survey of literature yielded an abundance of information on the technical treatment to remove nitrate from water including ion exchange [7], biological denitrification [8], [9], [10], chemical denitrification [11], [12], [13], catalytic denitrification [14], [15], reverse osmosis [16] and electrodialysis [17]. Current technologies for removal of nitrate like ion exchange, reverse osmosis are not selective to nitrate, generate secondary brine wastes and require frequent media regeneration.

Owing to above difficulties, hydrotalcite like compounds (HTlc) was thought to prove a potential adsorbent for the removal of nitrate. Hydrotalcite-like compounds constitute an important class of inorganic materials with desirable properties to remove anionic pollutants from water [18], [19], [20], [21], [22]. Hydrotalcites, also known as layered double hydroxides (LDHs) or ionic clays are based upon the brucite [Mg(OH)₂] structure in which some of the divalent cations are replaced by trivalent cations (e.g., Al, Fe, Cr etc.) resulting in a layer charge. This layer charge is counter balanced by anions such as carbonate, nitrate, sulfate or chloride in the interlayer spaces. In LDHs a broad range of compositions are possible of the type [M²⁺_1−xM³⁺_x(OH)₂][Aⁿ⁻]_x/n·yH₂O, where M²⁺ and M³⁺ are the divalent and trivalent cations in the octahedral positions within the hydroxide layers with x normally between 0.17 and 0.33. Aⁿ⁻ is an exchangeable interlayer anion. The degree of anionic exchange in the LDHs depends on the structural characteristics e.g. the nature of the interlayer anion and crystallinity. Exchange conditions like pH and carbonate contamination from environment are also important limitations. Due to the high affinity of LDHs toward carbonate ion, materials with intercalated carbonate ions have relatively smaller exchange capacities unless being calcined. High pH conditions must be applied to maintain the stability of LDH, however, OH⁻ intercalation is competitive in this case.

Although studies have examined the synthesis of various LDHs and the way in which they interact with various anions, the synthesis of Zn/Al chloride layered double hydroxide and its application towards removal of nitrate has not been examined previously. So, the present research was aimed to synthesize, characterize and to study the removal efficiency of nitrate by Zn/Al chloride LDH. The effect of various parameters on the effectiveness of treating nitrate contaminated water with Zn/Al chloride was unknown. Therefore, in this study, Zn/Al chloride was added to nitrate solutions and the effect of different variables (calcination temperature, dose, time, pH, initial nitrate concentration, effect of other anions etc.) on the removal of nitrate from solution by Zn/Al chloride was examined.

Section snippets

Reagents and chemicals

Potassium nitrate, zinc chloride, aluminum chloride, sodium chloride and sodium hydroxide used in the present study were of analytical grade and were obtained from Merck. 1000 mg/L stock solution of nitrate was prepared by dissolving 1.6305 g of KNO₃ in 1 L decarbonated distilled water. The required concentration of nitrate solution was obtained by serial dilution of 1000 mg/L nitrate solution. The measuring cylinder, volumetric flask and conical flask and other glassware used were of Borosil.

Synthesis of Zn–Al–Cl hydrotalcite

Effect of adsorbent dose

The effect of variation of adsorbent dose on percentage removal of nitrate from aqueous solution with LDH used in this study is graphically shown in Fig. 5. It is evident from the figure that the removal of nitrate increased from 66.8% to 85.5%, 61.4% to 80.4% and 55.5% to 75.7% for 0.1 to 0.8 g of LDH in 100 mL of synthetic nitrate solution of initial concentration, 10 mg/L, 50 mg/L and 100 mg/L respectively. However it is observed that after dosage of 0.3 g/100 mL, there was no significant change in

Conclusion

Zn–Al–Cl hydrotalcite exhibited much greater specific surface area and which increased with increase in calcination temperature. The adsorption of nitrate from aqueous solution by Zn–Al–Cl hydrotalcite was found to occur readily. Adsorption of nitrate was found to follow first order kinetics. The effect of other anions were also studied and was found that the anions reduced the nitrate adsorption in the order of carbonate > phosphate > chloride > sulphate. The exchange is favored for in-going anions

Acknowledgement

The authors are thankful to Prof. S.K. Sarangi, (Director) Prof. K.M. Purohit, and staff members of Department of Chemistry, National Institute of Technology, Rourkela, for providing necessary facilities and necessary help in carrying out the research work. The authors are also thankful to Prof. U.C. Patra, Director, Purushottam Institute of Engineering & Technology, Rourkela for his necessary help and cooperation.

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Synthesis and physicochemical characterization of Zn/Al chloride layered double hydroxide and evaluation of its nitrate removal efficiency

Abstract

Introduction

Section snippets

Reagents and chemicals

Synthesis of Zn–Al–Cl hydrotalcite

Effect of adsorbent dose

Conclusion

Acknowledgement

Int. J. Hyg. Environ. Health

Water Res.

Aquaculture

Water Res.

Appl. Catal. B: Environ.

J. Hazard. Mater.

J. Hazard. Mater.

J. Hazard. Mater.

J. Hazard. Mater.

Water Res.

Chem. Eng. Sci.

Catal. Today

Desal.

Micropor. Mesopor. Mater.

J. Nucl. Mater.

J. Colloid Interface Sci.

Sep. Purif. Technol.

J. Colloid Interface Sci.

J. Solid State Chem.