Nitrate removal from water using surface-modified adsorbents

University of Technology Sydney. Faculty of Engineering and Information Technology.Elevated concentrations of nitrate in surface and ground waters can cause eutrophication of natural water bodies, and in drinking water they can pose a threat to human health, especially to infants by causing ‘blue baby’ syndrome. Adsorption technology is an attractive method to remove nitrate from water compared to other technologies in terms of simplicity, cost, design, operation and maintenance, and effectiveness. An anion exchange resin known as Dowex 21K XLT was surface modified by incorporating Fe (Dowex-Fe) to increase the surface positive charges and tested for removing nitrate. The batch adsorption data at pH 6.5 fitted well to the Langmuir model with maximum adsorption capacities of 27.6 mg N/g, and 75.3 mg N/g for Dowex and Dowex-Fe resins, respectively. The fluidised-bed adsorption capacities were 18.6 mg N/g and 31.4 mg N/g at a feed concentration of 20 mg N/L and filtration velocity of 5 m/h for Dowex and Dowex-Fe, respectively. Low-cost agricultural wastes, specifically corn cob and coconut copra were also surface modified but by amine-grafting to increase the surface positive charges. The Langmuir nitrate adsorption capacities (mg N/g) were 49.9 and 59.2 for the amine-grafted (AG) corn cob and AG coconut copra, respectively, at pH 6.5. Fixed-bed adsorption capacities were 15.3 mg N/g and 18.6 mg N/g at the same feed concentration and flow velocity as in the Dowex study for AG corn cob and AG coconut copra, respectively. In both batch and column experiments, nitrate adsorption declined in the presence of sulphate, phosphate and chloride, with sulphate being the most competitive anion. More than 95% of adsorbed nitrate was desorbed by 1 M KCl in all adsorption/desorption cycles and the adsorbents were successfully regenerated in each cycle with little reduction in adsorption capacity. A submerged membrane (microfiltration) adsorption hybrid system (SMAHS) was utilised for the continuous removal of nitrate. The volume of water treated to maintain the nitrate concentration below the WHO limit of 11.3 mg N/L and the amount of nitrate adsorbed per gram of adsorbent for all four flux (2.5, 5, 10 and 15 L/m²h) tested were in the order Dowex-Fe > Dowex > AG coconut copra > AG corn cob. A rise in flux increased the volume of water treated and the amount of nitrate adsorbed. The exhausted agricultural waste adsorbents in both the column and SMAHS trials can be directly applied to lands as nitrate fertilisers, while the desorbed nitrate solution containing K can be used in fertigation to supply nutrients (N and K) to plants. An electrochemical-adsorption system was investigated to remove nitrate simultaneously using the adsorption and electrochemical methods. In this system four adsorbents were added inside an anode stainless steel box where the Cu plate served as the cathode. It was found that nitrate removal was higher in a short period of time and the cost was low. The optimum nitrate removal scenario for the integrated system was at pH 7, 1 A, and 31 V for a distance of 1 cm apart between the electrodes. Nitrate removal in the integrated system is approximately the sum of the removals derived from the individual processes. The innovative feature of this study is the integration of an electrochemical system with the adsorption process where the adsorbents are kept intact with the anode. The different methods undertaken in the four nitrate removal studies can’t be compared and each method has advantages and disadvantages in terms of nitrate removal efficiency, cost, raw water quality and removal efficiency of other pollutants. However, if the raw water contains only nitrate the column method is best compared to other methods. It is recommended that the encouraging results obtained in our laboratory scale studies be tested in series of cells connected to each other for continuous removal of nitrate. It is also recommended that these experiments are conducted at pilot plant scale, which is closer to practical conditions

Mathematical modelling of nitrate removal from water using a submerged membrane adsorption hybrid system with four adsorbents

© 2018 by the authors. Excessive concentrations of nitrate in ground water are known to cause human health hazards. A submerged membrane adsorption hybrid system that includes a microfilter membrane and four different adsorbents (Dowex 21K XLT ion exchange resin (Dowex), Fe-coated Dowex, amine-grafted (AG) corn cob and AG coconut copra) operated at four different fluxes was used to continuously remove nitrate. The experimental data obtained in this study was simulated mathematically with a homogeneous surface diffusion model that incorporated membrane packing density and membrane correlation coefficient, and applied the concept of continuous flow stirred tank reactor. The model fit with experimental data was good. The surface diffusion coefficient was constant for all adsorbents and for all fluxes. The mass transfer coefficient increased with flux for all adsorbents and generally increased with the adsorption capacity of the adsorbents

Enhanced removal of nitrate in an integrated electrochemical-adsorption system

© 2017 Elsevier B.V. The electrochemical (EC) method of removing pollutants in water is a widely used process in water and wastewater treatment. An EC-adsorption integrated system was investigated to test whether the simultaneous removal of nitrate by the two processes would be better than removal utilising the individual EC and adsorption methods. In the integrated system, an adsorbent (ion exchange resin - Dowex 21k XLT) was placed inside a stainless steel box that served as an anode with a Cu plate as cathode. In an experiment using 2 L nitrate solution containing 20 mg N/L and 2 g adsorbent the rate of nitrate removal in the integrated system was initially fast with 35% removed in 30 min, though slowing down later. The rate of removal increased with increasing current, voltage and pH up to 7 but decreased as the distance between the electrodes also increased. The optimum nitrate removal of 67% was obtained at pH 7, 1 A, and 31 V for a distance of 1 cm between the electrodes after 180 min. The amount of nitrate removed fell when sulphate was present in the integrated system due to sulphate competing with nitrate for adsorption. Concentration of ammonium produced by nitrate reduction in the EC system was reduced in the presence of adsorbent. Nitrate removal in the integrated system is approximately equal to the sum of the removals in the two individual processes

Modelling equilibrium adsorption of single, binary, and ternary combinations of Cu, Pb, and Zn onto granular activated carbon

© 2018, Springer-Verlag GmbH Germany, part of Springer Nature. Elevated concentrations of heavy metals in water can be toxic to humans, animals, and aquatic organisms. A study was conducted on the removal of Cu, Pb, and Zn by a commonly used water treatment adsorbent, granular activated carbon (GAC), from three single, three binary (Cu-Pb, Cu-Zn, Pb-Zn), and one ternary (Cu-Pb-Zn) combination of metals. It also investigated seven mathematical models on their suitability to predict the metals adsorption capacities. Adsorption of Cu, Pb, and Zn increased with pH with an abrupt increase in adsorption at around pH 5.5, 4.5, and 6.0, respectively. At all pHs tested (2.5–7.0), the adsorption capacity followed the order Pb > Cu > Zn. The Langmuir and Sips models fitted better than the Freundlich model to the data in the single-metal system at pH 5. The Langmuir maximum adsorption capacities of Pb, Cu, and Zn (mmol/g) obtained from the model’s fits were 0.142, 0.094, and 0.058, respectively. The adsorption capacities (mmol/g) for these metals at 0.01 mmol/L equilibrium liquid concentration were 0.130, 0.085, and 0.040, respectively. Ideal Adsorbed Solution (IAS)-Langmuir and IAS-Sips models fitted well to the binary and ternary metals adsorption data, whereas the Extended Langmuir and Extended Sips models’ fits to the data were poor. The selectivity of adsorption followed the same order as the metals’ capacities and affinities of adsorption in the single-metal systems

Iron-impregnated granular activated carbon for arsenic removal: Application to practical column filters

© 2019 Elsevier Ltd Arsenic is a major drinking water contaminant in many countries causing serious health hazards, and therefore, attempts are being made to remove it so that people have safe drinking water supplies. The effectiveness of arsenic removal from As(V) solutions using granular activated carbon (GAC) (zero point of charge (ZPC) pH 3.2) and iron incorporated GAC (GAC-Fe) (ZPC pH 8.0) was studied at 25 ± 1 °C. The batch study confirmed that GAC-Fe had higher Langmuir adsorption capacity at pH 6 (1.43 mg As/g) than GAC (1.01 mg As/g). Adsorption data of GAC-Fe fitted the Freundlich model better than the Langmuir model, thus indicating the presence of heterogeneous adsorption sites. Weber and Morris plots of the kinetic adsorption data suggested intra-particle diffusion into meso and micro pores in GAC. The column adsorption study revealed that 2–4 times larger water volumes can be treated by GAC-Fe than GAC, reducing the arsenic concentration from 100 μg/L to the WHO guideline of 10 μg/L. The volume of water treated increased with a decrease in flow velocity and influent arsenic concentration. The study indicates the high potential of GAC-Fe to remove arsenic from contaminated drinking waters in practical column filters

Enhanced removal of nitrate from water using amine-grafted agricultural wastes

© 2016 Elsevier B.V. Adsorption using low-cost adsorbents is a favourable water treatment method for the removal of water contaminants. In this study the enhanced removal of nitrate, a contaminant at elevated concentration affecting human health and causing eutrophication of water, was tested using chemically modified agricultural wastes as adsorbents. Batch and fixed-bed adsorption studies were performed on corn cob and coconut copra that were surface modified by amine-grafting to increase the surface positive charges. The Langmuir nitrate adsorption capacities (mg N/g) were 49.9 and 59.0 for the amine-grafted (AG) corn cob and coconut copra, respectively at pH 6.5 and ionic strength 1 × 10-3 M NaCl. These values are higher than those of many commercially available anion exchange resins. Fixed-bed (15-cm height) adsorption capacities (mg N/g) calculated from the breakthrough curves were 15.3 and 18.6 for AG corn cob and AG coconut copra, respectively, for an influent nitrate concentration 20 mg N/L at a flow velocity 5 m/h. Nitrate adsorption decreased in the presence of sulphate, phosphate and chloride, with sulphate being the most competitive anion. The Thomas model fitted well to the fixed-bed adsorption data from four repeated adsorption/desorption cycles. Plug-flow model fitted well to the data from only the first cycle

Removing polycyclic aromatic hydrocarbons from water using granular activated carbon: kinetic and equilibrium adsorption studies

© 2018, Springer-Verlag GmbH Germany, part of Springer Nature. Polycyclic aromatic hydrocarbons (PAHs) constitute a group of highly persistent, toxic and widespread environmental micropollutants that are increasingly found in water. A study was conducted in removing five PAHs, specifically naphthalene, acenaphthylene, acenaphthene, fluorene and phenanthrene, from water by adsorption onto granular activated carbon (GAC). The pseudo-first-order (PFO) model satisfactorily described the kinetics of adsorption of the PAHs. The Weber and Morris diffusion model’s fit to the data showed that there were faster and slower rates of intra-particle diffusion probably into the mesopores and micropores of the GAC, respectively. These rates were negatively related to the molar volumes of the PAHs. Batch equilibrium adsorption data fitted well to the Langmuir, Freundlich and Dubinin–Radushkevich models, of which the Freundlich model exhibited the best fit. The adsorption affinities were related to the hydrophobicity of the PAHs as determined by the log Kow values. Free energies of adsorption calculated from the Dubinin–Radushkevich model and the satisfactory kinetic data fitting to the PFO model suggested physical adsorption of the PAHs. Adsorption of naphthalene, acenaphthylene and acenaphthene in fixed-bed columns containing a mixture of GAC (0.5 g) + sand (24.5 g) was satisfactorily simulated by the Thomas model

Submerged membrane adsorption hybrid system using four adsorbents to remove nitrate from water

© 2017, Springer-Verlag Berlin Heidelberg. Nitrate contamination of ground and surface waters causes environmental pollution and human health problems in many parts of the world. This study tests the nitrate removal efficiencies of two ion exchange resins (Dowex 21K XLT and iron-modified Dowex 21K XLT (Dowex-Fe)) and two chemically modified bio-adsorbents (amine-grafted corn cob (AG corn cob) and amine-grafted coconut copra (AG coconut copra)) using a dynamic adsorption treatment system. A submerged membrane (microfiltration) adsorption hybrid system (SMAHS) was used for the continuous removal of nitrate with a minimal amount of adsorbents. The efficiency of membrane filtration flux and replacement rate of adsorbent were studied to determine suitable operating conditions to maintain the effluent nitrate concentration below the WHO drinking standard limit of 11.3 mg N/L. The volume of water treated and the amount of nitrate adsorbed per gramme of adsorbent for all four flux tested were in the order Dowex-Fe > Dowex > AG coconut copra > AG corn cob. The volumes of water treated (L/g adsorbent) were 0.91 and 1.85, and the amount of nitrate removed (mg N/g adsorbent) were 9.8 and 22.2 for AG corn cob and Dowex-Fe, respectively, at a flux of 15 L/(m2/h)

Removing nitrate from water using iron-modified Dowex 21K XLT ion exchange resin: Batch and fluidised-bed adsorption studies

© 2015 Elsevier B.V. All rights reserved. Elevated concentrations of nitrate in surface waters can cause eutrophication, while in drinking water they pose a threat to human health, especially causing blue baby syndrome in infants. An anion exchange resin - Dowex 21K XLT - was surface modified by incorporating Fe (Dowex-Fe) and tested to remove nitrate from aqueous solutions in batch and fluidised-bed adsorption experiments. Solution pH in the 4.0-7.5 range had no effect on nitrate adsorption. The adsorption data at pH 6.5 fitted well to the Langmuir model with maximum adsorption capacities of 27.6 mg N/g, and 75.3 mg N/g for Dowex and Dowex-Fe resins, respectively. Energy dispersion spectrometry and zeta potential measurements indicated that the increase in adsorption due to Fe impregnation was caused by additional surface positive charges induced on the resin by Fe. In both batch and fluidised-bed experiments, nitrate adsorption capacity declined markedly when sulphate was added, but phosphate and chloride additions had little effect. Batch kinetic data fitted well to the pseudo-first, pseudo-second and homogeneous surface diffusion models. Data from the fluidised-bed experiments satisfactorily fitted to the Thomas and plug-flow models. More than 95% of adsorbed nitrate was desorbed by 1 M KCl in all three adsorption/desorption cycles and the resins were successfully regenerated in each cycle with little reduction in adsorption capacity. No significant reduction in the Fe content of Dowex-Fe occurred during the regeneration