Removal of Harmful Anions in Aqueous Solution with Various Layered Double Hydroxides

Abstract

Four types of layered double hydroxides (LDHs) were synthesized by a co-precipitation method. The ability of various LDHs to remove anionic species including As(III), As(V), B, Cr(VI) and Se(IV) was investigated. Removal tests were performed primarily by a batch operation. The Cr(VI) removal was conducted by both batch and column studies. For the column removal tests, LDH pellets were prepared using a granulator and 5% poly-vinyl alcohol solution as binder.Among the LDHs tested, Mg-Al-NO 3 − LDH showed the optimal removal of anionic species, and it demonstrated excellent removal of As(V), Se(IV) and Cr(VI) particularly at the low anionic concentrations. The order of anionic removal by the LDHs is as follows; Mg-Al-NO 3 − LDH > Mg-AlCl − LDH >> Mg-Al-SO 4 2− LDH > Mg-Al-CO 3 2− LDH. The selective uptake of anion had the following order; As(V) > Se(IV) > Cr(VI) > As(III) > B. This study has demonstrated the uptake of harmful anions is influenced by the two specific factors, (1) the originally intercalated anions in the LDH, and (2) valency of anionic target species in aqueous solution. Column study also demonstrated that about 350 times Cr(VI) in solution per unit volume of LDH pellet can be removed, though this resulted in minor change in pH of the solution and greater NO 3 − elution. This study has revealed the ability of LDH to remove harmful anionic species and its practical application in wastewater treatment.

Anion Selectivity of Layered Double Hydroxides: Effects of Crystallinity and Charge Density

“…[31] In fact, carbonate was detected as listed in Table 1. Up to a Mg/Al ratio of 3 for COP-LDHs and 4 for HYDLDHs at the preparation stage, the total charges of Cl -and CO 3 2-are almost equal to the molar amounts of Al, but at higher Mg/Al ratios, the total charges of Cl -and CO 3 2-exceed the molar amounts of Al. Formation of a kind of basic magnesium carbonate on the external surface may be one of the causes.…”

“…[ [21][22][23][24][25][26] In those previous studies, only a limited range of the M 2+ /M 3+ molar ratios was investigated, the effect of crystallinity or LDH size was only insufficiently examined, and the behavior of monovalent anions other than NO 3 -was not clarified. In addition, small LDHs, with crystallite dimensions of ca.…”

“…[9,10,[13][14][15][16][17][18] However, the selectivity of LDHs for nitrate is generally considered not to be high, and a large number of studies have reported that anions with a higher charge density tend to be more preferentially intercalated into LDHs. [6][7][8]19,20] According to a report by Miyata, [19] the order of exchange selectivity for monovalent anions in the case of Mg-Al LDHs (Mg/ Al = 2.3) is OH -> F -> Cl -> Br -> NO 3 -> I -, and according to a report by Costa et al, [20] the order of exchange selectivity based on a computational simulation with Zn-Al LDHs (Zn/Al = 2) is CO 3 2-> OH -> F -> Cl -> Br -> NO 3 -. It is therefore very questionable whether LDHs could be used as nitrate adsorbents.…”

“…In the case of F -, the diameter of which is smaller than the height of the flat orientation of NO 3 -, this result indicates that an ion sieve effect is not the only factor that determines selectivity. Other factors, such as the interactions of NO 3 -with the interlayer surface and with interlayer water, may also be involved.…”

“…[1][2][3][4][5] LDHs are a class of lamellar materials in which metal hydroxide layers lie on top of each other, and anions and water molecules are located in the interlayer space. Although a wide variety of LDHs have been reported in terms of the constituent metals, the general formula of the LDHs that have most commonly been studied can be written as [M , and M 3+ are diand trivalent metal cations, and A n-refers to charge-balancing anions, which can often be a group of different anions.…”