*Article* **Evaluation of Adsorption Mechanism of Chromium(VI) Ion Using Ni-Al Type and Ni-Al-Zr Type Hydroxides**

**Fumihiko Ogata <sup>1</sup> , Noriaki Nagai <sup>1</sup> , Ayako Tabuchi <sup>1</sup> , Megumu Toda <sup>2</sup> , Masashi Otani <sup>2</sup> , Chalermpong Saenjum 3,4 , Takehiro Nakamura <sup>1</sup> and Naohito Kawasaki 1,5,\***


**Abstract:** To evaluate the feasibility of nickel–aluminum (the Ni2+:Al3+ molar ratios of 1.0:1.0 and 1.0:2.0 are denoted as NA11 and NA12, respectively) and nickel–aluminum–zirconium type (the Ni2+:Al3+:Zr4+ molar ratios of 0.9:1.0:0.09 and 0.9:2.0:0.09 are denoted as NAZ1 and NAZ2, respectively) hydroxides for Cr(VI) removal from aqueous media, the adsorption capability and adsorption mechanism of Cr(VI) using the above-mentioned adsorbents were investigated in this study. The quantity of Cr(VI) adsorbed onto NA11, NA12, NAZ1, and NAZ2 was 25.5, 25.6, 24.1, and 24.6 mg g−<sup>1</sup> , respectively. However, the quantity of aluminum (base metal) released from NA11 (approximately 0.14 mg g−<sup>1</sup> ) was higher than that from NAZ1 (approximately 1.0 µg g−<sup>1</sup> ), indicating that NAZ1 was more suitable for Cr(VI) removal than NA11. In addition, the effects of pH, contact time, and temperature on the adsorption of Cr(VI) were evaluated. Moreover, to elucidate the adsorption mechanism of Cr(VI) using NA11 and NAZ1, the elemental distribution, X-ray photoelectron spectrometry spectra, and ion exchange capability were also determined. Cr(VI) adsorbed onto the NAZ1 surface was easily desorbed using a sodium hydroxide solution under our experimental conditions. The information regarding this study can be useful for removing Cr(VI) from aqueous media.

**Keywords:** nickel–aluminum complex hydroxide; nickel–aluminum–zirconium complex hydroxide; chromium(VI); adsorption
