Synthesis of nanosized ZSM-5 zeolite from rice straw using lignin as a template: Surface-modified zeolite with quaternary ammonium cation for removal of chromium from aqueous solution

Extracted silica and lignin from local rice straw were respectively used as a source of silica and a templating agent for the synthesis of nanosized ZSM-5 zeolite at different aging temperatures with the autoclave process. A comparative analysis between synthesized ZSM-5 with lignin (ZL) and its respective synthesized with most common tetrapropyl-ammonium bromide (TPABr) (ZT) using XRD, FT-IR, TEM, TGA/DTGA and pore structure analysis by N2 adsorption at −196 °C was conducted. A higher degree of crystallinity and a larger specific surface area (SBET) were presented for ZL at aging temperature of 160 than at 140 °C. Concurrently, a decrease in intensity of the diffraction line at d = 3.342 Ǻ due to α-quartz silica was seen, giving rise to an increase of the dissolution rate of silica component in the gel phase at 160 over 140 °C. Complementary TEM and XRD techniques informed that the particle sizes measured 50 and 75 nm for the former and latter, respectively. The ZL surface was modified with TPA+ (50–200 mmol) and applied for chromate anions (5–30 ppm) adsorption experiments in the aqueous solution at room temperature and pH 5. The capacity of chromate adsorption over ZL-modified TPA+ was greater by threefold than over ZL by the combination of electrostatic effects. The kinetics obeyed Freundlich model where the adsorption onto ZL-modified TPA+ affords 3.7 mmol/g.

The sorption of chromates on surfactant-modified ZL (SMZ) with different amounts of TPABr surfactant was investigated under various conditions for the purpose of removing CrVI–oxyanions from aqueous media. This can be explained by asserting that SMZ is charged positively with TPA+ of the structure NR4+, R being an alkyl group, which is stabilized by surface active binding sites of silanol groups in ZSM-5. The positively charged ‘‘head’’ groups are balanced by the bromide counter ions, and the adsorption of chromate anions brings about the substitution of these weakly bonded counter ions. Thus, TPA+ presents positively charged functional groups towards the solution, which can serve as sorption sites for chromate-saturated SMZ.Figure optionsDownload as PowerPoint slideHighlights
► Extracted lignin from rice straw is used for synthesis of nanosized ZSM-5 zeolite.
► The gel aging at 160 °C yielded the highest degree of crystallinity.
► The zeolite-modified TPA+ possesses an enhanced chromium sorption capacity.
► The solution pHs control the nature of the anchorage of the chromate moieties.