Selective nitrate removal from aqueous solutions by a hydrotalcite-like absorbent FeMgMn-LDH

Abstract

FeMgMn-LDH, a type of potential environmental remediation material, has been synthesized via a co-precipitation method, and its adsorption characteristics for nitrate were investigated in this study. It’s shown that the prepared FeMgMn-LDH is a promising adsorbent for anions removal, which has high buffer capacity (final pH remained between 9 and 10) and high reversibility, and can remove nitrate ions selectively though an anion-sieve effect. The maximum amount of nitrate adsorption is 10.56 N-mg g−1 at 25 ℃. The removal rate of nitrate ions can reach 86.26% with the adsorbent dose of 5 g/L in a real water. The competition order of coexisting anions on nitrate adsorption by FeMgMn-LDH is CO32− > PO43− > SO42−. The negative values of ΔG0 (from − 27.796 to − 26.426 kJ mol−1) and ΔH0 (− 6.678 kJ mol−1) indicate that the nitrate adsorption process on the FeMgMn-LDH is spontaneous and exothermic. The main adsorption mechanisms of nitrate removal from aqueous solutions by FeMgMn-LDH are electrostatic attraction and ion exchange.

Adsorption of nitrate from interflow by the Mg/Fe calcined layered double hydroxides

“…The parameter of maximum adsorption capacity (q m ) fitted by the Langmuir model is 73.36 mg/g, which was higher than the commonly used nitrate adsorbents such as activated carbon (14.3 mg/g) (Xia et al 2020), biomass (30.21 mg/g) (Kilpimaa et al 2015), biochar (24.8 mg/g) (Chen et al 2020) and clay minerals (5.1 mg/g) (Battas et al 2019). Moreover, this q m was also greater than some other LDHs such as Mg/Fe-Cl LDH (18.17 mg/g) (Santos et al 2020), Fe/Mg/ Mn-LDH (46.77 mg/g) (Zhou et al 2020) and Mg/Al LDH (28.06 mg/g) (Alagha et al 2020a). Notably, the LDHs with special modifications may exhibit higher adsorption capacity.…”

“…This phenomenon was mainly due to the fact that CLDHs have a higher affinity toward anions with a relatively high charge density, thus divalent anions have a more apparent inhibitory effect on the adsorption by CLDHs compared with monovalent anions (Yang et al 2015;Tong et al 2017). Moreover, the inhibitory effect of CO 3 2À is more obvious than that of SO 4 2À , which may be explained by the ion-sieve effect (Zhou et al 2020). Studies have reported that the adsorption towards anions by CLDHs is simultaneously controlled by physical interactions, that is, the selectivity of anions is usually influenced by the size of anions, named ion-sieve effect caused by the spatial limits of the CLDH brucite layers (Halajnia et al 2012).…”

“…For example, Tezuka et al (2005) have proved the high selectivity for nitrate by Ni/Fe LDH owing to the suitable interlayer spacing. Since CO 3 2À (0.28 Â 0.15 nm) has a similar ion size with nitrate (0.29 Â 0.15 nm), CO 3 2À can also compete with nitrate for adsorption sites through ion-sieve effect (Zhou et al 2020). Therefore, divalent CO 3 2À has a more obvious inhibitory effect in comparison to divalent SO 4 2À on nitrate adsorption.…”

Layered Double Hydroxides as Rising-Star Adsorbents for Water Purification: A Brief Discussion

“…are considered dangerous to humans and wildlife, even at very low concentrations [ 17 , 49 ]. In the case of oxyanions, the primarily reported mechanisms are ion exchange, electrostatic attraction, and coordination [ 17 , 50 ].…”

“…Zhou et al [ 50 ] prepared FeMgMn-LDH via the co-precipitation method for the removal of nitrate from an aqueous solution. The surface area of the material was 47 m 2 ·g −1 ; the presence of –OH, N=O, M–OH and M–O–M was detected in its structure.…”