Sorption behavior of methyl orange from aqueous solution on organic matter and reduced graphene oxides modified Ni–Cr layered double hydroxides

• Synthesis of Ni–Cr–CO3 LDH, RGO–Ni–Cr–CO3 LDH, Ni–Cr–DBS LDH and Ni–Cr–LA LDH.
• Organic Ni–Cr LDH shows larger sorption amount for high concentration MO.
• Large specific area of RGO–Ni–Cr–CO3 LDH and the bridge of RGO and MO in the presence of heavy metals may enhance sorption.
• MO sorption is on the external surface of organic Ni–Cr LDH and affected by the partition into the interlay organic phase formed by organic anions.

Nickel chromium layered double hydroxides (LDH) modified by reduced graphene oxide (RGO–Ni–Cr–CO3 LDH), sodium dodecyl benzene sulfonate (Ni–Cr–DBS LDH), laurate (Ni–Cr–Laurate LDH) as well as the typical carbonate-intercalated Ni–Cr LDH (Ni–Cr–CO3 LDH) were synthesized by co-precipitation method and characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and elemental analysis (EA). The results of the batch sorption and recycle experiments of Ni–Cr LDHs towards methyl orange (MO) show the great potential of three modified Ni–Cr LDHs as good sorbents. The adsorption kinetics curves of MO onto the Ni–Cr LDHs were better described by the pseudo-second-order model than pseudo-first-order. The adsorption capacity follows the order of Ni–Cr–DBS LDH > Ni–Cr–Laurate LDH > RGO–Ni–Cr–CO3 LDH >> Ni–Cr–CO3 LDH. Sorption isotherms of the inorganic LDHs (RGO–Ni–Cr–CO3 LDH and Ni–Cr–CO3 LDH) are Langmuir type, while LDHs intercalated with organic molecules fitted well with Freundlich type. Greater sorption of MO on RGO–Ni–Cr–CO3 LDH is attributed to the large specific area and the bridge of RGO and MO in the presence of heavy metals. The partition into the interlay organic phase formed by organic anions (DBS or laurate), and the adsorption onto the external surface of LDHs may dominated the progress of MO sorption on Ni–Cr–DBS LDH and Ni–Cr–Laurate LDH. The regeneration results showed the as-modified Ni–Cr LDHs could be regarded as a promising adsorbent for the removal of anionic dyes in wastewaters.

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