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.