Synthetic nanoclays for use in water remediation: synthesis, characterization, and applications

GIPSY PEÑA RAMIREZ; LEONARDO ANDRES CANO; VERA ALVAREZ

Campinas

Congreso; Sao Paulo School for Advanced Science in Nanotechnology, Agriculture, and the Environment; 2023

The Brazilian Center for Research in Energy and Materials (CNPEM)

Significant increase in nitrate levels in groundwater has been observed in many countries related to anthropic activities, as extensive agriculture, excessive application of nitrogenous fertilizers and inadequate wastewater treatment [1]. Nitrate is a potential human health hazard, especially to infants, causing the condition known as met-hemoglobinemia, also called blue baby syndrome [2]. Chronic accumulate of high level of nitrate and nitrite may also cause other health problems, for example some cancers and teratogenic effects [3]. Conventional treatments are not effective in eliminating these contaminants, the current alternatives for treatment are ionic exchange, reverse osmosis and electro-dialysis, however, these technologies are expensive, generate charged effluents and have a complex operation maintenance. For this reason, economic, effective and easy-to-use alternatives are required. Layered Double Hydroxides (LDH) are clay minerals that have properties such as high specific surface area, two-dimensional structure, ion exchange capacity, surface charge and high memory effect, [4] which allow HDL to be used as adsorbents for contaminants in water treatment. The synthesis of these nanoclays was carried out by coprecipitation, using three solutions (1.5M Mg/Al solution in a 3:1 ratio, another 1M Na2CO3 and 2M NaOH), subsequently the solid obtained was filtered, washed, dryed and calcined at 500°C. These solid was characterized by X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). The XRD showed characteristic signals of hydrotalcite-type materials (Standard ref. 22-0700, JCPDS), showing for the synthesized LDH, fine and intense peaks for asymmetric flexions in the planes (003), (006) and (009), wide and with less intensity in the planes (015) and (018), and signals in planes (110) and (113). The XRD for the calcined LDH sample presents only two peaks corresponding to planes (200) and (220), evidencing the conversion of LDH into a mixture of oxides through calcination. For the LDH used, similar signals at fresh LDH were presented, with a slightly lower intensity, demonstrating the regeneration of the structure due to the adsorption of nitrates, after its calcination and use. The FTIR spectrum for the fresh LDH showed a wide absorption band between 3300-3700cm-1 due to the stretching of the OH groups present in the brucite-type sheets, another band at 1643cm-1 coming from the interlaminar water, a band at 1367cm-1 from the NO3- present in the interlaminar space and between 400-700cm-1 bands attributed to characteristic stretching and bending of hydroxyl groups attached to Al and Mg are observed. For calcined LDH, the signals corresponding to adsorbed water and nitrates are reduced, this is due to the collapse of the clay structure. For the LDH used, it is observed that the characteristic signals that were present in the fresh LDH are maintained, being the most intense band at 1367cm-1, due to the adsorption of nitrates in the structure. The removal tests carried out on 100 ppm solutions and with an LDH dose of 100mg/50ml, showed removal percentages of nitrates between 94.58 and 99.23% and adsorption capacity (q) between 46.97 and 49.47mg/g, proving to be promising materials for nitrate removal.