3.1. Nickel Molybdate Nanosorbent Preparation

All synthetic compounds aside from the methylene blue (provided by Panreac, Barcelona, Spain) were purchased from Sigma-Aldrich (St. Louis, MO, USA) and utilized as received with no alterations.

Nickel molybdate (NiMoO₄) was formed by thermal breakdown of a nickel molybdenum complex obtained from the reaction of oxalic acid dihydrate H₂C₂O₄·2H₂O, nickel nitrate Ni(NO₃)₂·6H₂O, and ammonium molybdate (NH₄)₆Mo₇O₂₄·4H₂O in its solid form, as defined previously in the literature [32]. Nickel nitrate, oxalic acid dihydrate, and ammonium molybdate were blended jointly together in a molar proportion of 1/10/0.143. The mixture was powdered homogeneously and placed on a hot plate at a temperature of 160 °C for heating. The obtained nickel molybdenum complex was then decomposed under the control with static air at 500 °C for 2 h inside a cylindrical furnace, which was open at the two ends.