Magnetically controllable sponges for crude oil, mercury and arsenic removal

Abstract

In this work, we present three different pathways to render commercial melamine sponges, both magnetic and hydrophobic, thereby offering them the capacity to effectively and selectively remove crude oil and heavy metals from aqueous environments. The magnetic properties were endowed by the deposition of as-synthesized iron oxide nanoparticles, functionalized with hydrophobic oleic acid and oleylamine ligands, via a large-scale synthesis. Aiming for an even higher hydrophobicity, terminal vinyl groups were attached to the pristine and modified sponges through a sol-gel hydrolysis of VTES@SiO2. The magnetic materials were thoroughly characterized and evaluated for their efficiency in water purification applications, regarding the absorption of crude oil and heavy metal pollutants. Each pathway of preparation was effectively practical for a different application. The initial coating of the sponges with a hydrophobic layer (Sp-h-m) enhanced the absorption and the retention of iron oxide nanoparticles, resulting at materials with a maximum magnetization of 25 x 10-3 emu. This modified sponge also exhibited the highest removal of metal ions (As3+ and Hg2+) from aqueous solutions within 60 minutes, and its extraction efficiency was evaluated in systems with single metal ions (250 ppm) and in the removal of metal ions (As3+ and Hg2+) from aqueous solutions within the same timeframe. It was evaluated for its extraction efficiency in systems with single metal ions (250 ppm) and under competitive conditions (500 ppm of toxic metals in total). The coating with a hydrophobic layer, following the deposition of the magnetic nanoparticles (Sp-m-h), led to further improvement of the sponge’s hydrophobicity (from 129o to 140o WCA), excellent selectivity to crude oil, and water repellency. The magnetically modified sponges exhibited significant initial absorption capacities (60–100 g/g) and average absorption capacities (35–65 g/g) over 15 cycles. The high selectivity, absorption/desorption efficiency (up to 99.8%), their absorption capacity for As and Hg metal ions and their responses upon external magnetic fields confirmed the suitability of the developed magnetic sponges for water purification systems. The versatility of the proposed modification route allows the preparation and optimization of specific magnetic sponges for targeted applications.