Efficient nitrate removal from water by biosorption onto a southern Mediterranean Sea native plant Posidonia oceanica (L.) powder

An efficient method for nitrate removal from water was developed using the sea plant Posidonia oceanica (L.) as a biosorbent. Never used previously for this purpose, this plant was applied in its natural form without any chemical modification or cross-linking with another material. The nitrate biosorption was carried out in batch experiment at room temperature and under the optimum conditions of agitation time, biosorbent dose, and pH. The Langmuir model fits the experimental biosorption isotherm data better than the Freundlich one, confirming a monolayer adsorption process onto homogeneous surface and giving a relatively high maximum biosorption capacity 41.6 mg <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>·</mml:mo> </mml:math> g <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow/> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> of nitrate nitrogen, thus demonstrating a good efficiency of the biosorbent. The thermodynamic study shows not only that the biosorption process is spontaneous and endothermic in nature, but also that increasing temperature enhances the nitrate uptake. Furthermore, the Dubinin–Radushkevich modeling gives a mean free energy of 15.81 kJ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>·</mml:mo> </mml:math> mol <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow/> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> , indicating that nitrate was chemisorbed. The same result was confirmed by kinetic modeling, which showed that the pseudo-second-order model fits the experimental data better than the pseudo-first order. The characterization by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) revealed that nitrate was mainly retained on the biosorbent surface through electrostatic forces with protonated amines ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi mathvariant="normal">RNH</mml:mi> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mo>+</mml:mo> </mml:msubsup> </mml:math> ; <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi mathvariant="normal">R</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:msubsup> <mml:mi mathvariant="normal">NH</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mo>+</mml:mo> </mml:msubsup> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi mathvariant="normal">R</mml:mi> <mml:mn>3</mml:mn> </mml:msub> <mml:msup> <mml:mi mathvariant="normal">NH</mml:mi> <mml:mo>+</mml:mo> </mml:msup> </mml:mrow> </mml:math> ). However, ion exchange with hydroxide ions and intraparticle diffusion may play an important role in the biosorption process.