Boron removal using Li-Al-OH layered double hydroxide prepared by one-step mechanochemical approach

In this study, Li-Al-OH layered double hydroxide (LDH), which was prepared by solvent-free one-step mechanochemical reaction of LiOH and Al(OH)3, was applied to remove boron from aqueous solution. Dry-grinding for 3 h at a rotational speed of 500 rpm, Li/Al molar 1/2 was the optimum condition to prepare highly crystalline of Li-Al LDH phase with no evident impure phases. Two milling products with Li/Al molar ratio at 1/2 and 2/2 were evaluated for boron adsorption. The results confirmed that Li/Al molar ratio 2/2 sample showed high boron adsorption capacity due to the physical adsorption of Li-Al-OH LDH and chemical synergism of phase gel Al(OH)3. The adsorption isotherms, described by the Langmuir model, indicated maximum monolayer boron uptake capacity 45.45 mg/g, implying competitive adsorption capacity of the material in our experiment.


Introduction
Boron naturally exists extensively in earth's hydrosphere and lithosphere [1].In nature, boron is always found as compounds combining with oxygen and other elements.Boron compounds were widely used in many industries, such as glass, electronics, ceramics, porcelain, cosmetics, semiconductors, leather, pharmaceuticals, insecticides, catalysts, fuel, and cleaning products [2].The glass industry was the biggest consumer among them, which consumes more than half of the total production of boron compounds [1].Moreover, boron was also an essential or at least a beneficial micronutrient for plants, human beings and animals [2].However, it became toxic when the amount of boron was slightly greater than required, and toxicity effects caused by excess boron were more common than boron deficiency in the environment.Since a series of environmental and health issues have been found caused by boron, in 2011, WHO revised the guideline value of boron to 2.4 mg/L in drinking water [1].However, only a few of countries followed the WHO recommendation, bcause the value of 2.4 mg/L exceeded the tolerate concentration of many crops.Therefore, in most cases lower boron concentrations could be acceptable, consequently, boron removal is a challenging problem.In order to obtain fresh water for drinking and irrigation, electrocoagulation [3], chemical precipitation [4], reverse osmosis [5], adsorption [6] have been used to boron recycling in the solution.Boron removing by adsorption process possesses low cost, adaptability, high separating efficiency advantages [1].Various sorbents has been utilized in adsorption processes for boron removal, including activated carbon, fly ash, natural minerals, layered double hydroxides (LDHs), biological materials, oxides, mesoporous silica, complexing membranes and selective resins [1].LDHs have aroused great attention in recent years due to their specific lamellar structure, high surface area, acidic-basic buffering capacity, high ion-exchange capacity, economic and versatile [7].Frederick et al. [8] reviewed boron removal by LDH and confirmed its potential wide use in purification of water in boron removal.
Layered double hydroxides (LDHs) are a group of synthetic anionic clays possessing a general formula as: [M 2+ 1-x M 3+ x(OH)2] x+ A y-x/y•nH2O, where M 2+ and M 3+ are divalent and trivalent metal cations, respectively, and A y− represents the interlayer anions [9].These materials have wide potential applications in various fields, including environmental protection, pharmaceutical preparation, organic synthesis, elastomer compositing, etc [10].
Among the numerous LDHs, Li/Al LDH is the only reported M(I)M(III) LDH which has the formula [LiAl2(OH)6] + A y− 1/y•nH2O.As such, the layer charge density of Li/Al LDH is the highest of all LDHs.Thus Li/Al LDH is superior to M(II)/M(III) LDHs because of its higher anion-exchange capacity.The uses of Li/Al LDH as adsorbents for the removal of contaminants especially heavy metals from wastewaters have been strongly addressed.Li-Al LDH, have been reported high adsorption efficiency toward Cr(V) because of the highest layer charge density among LDH compounds [11], but there are limited reports on the adsorption potential of Li/Al LDH in boron removal.
Li-Al LDH can be synthesized in aqueous solution operation including co-precipitation, hydrothermal method or urea decomposition-homogeneous precipitation [12].However, solution operation mentioned above may result in some problems, such as the formation of intermediate phases as impurities and increasing cost of production.Alternative processes to manufacture Li-Al LDH are required with the purpose to realize a much more environment-friendly process without emission of large amounts of wastes.Mechanochemical treatment to synthesize LDHs reported by William et al. [13] is an entire solvent-free approach to synthesis LDHs.It is promising to manufacture LDHs in large scale by mechanochemical approach because of their easy operation, energy-saving, and no waste water emission.Mg-Al-OH [13], Mg-Al-NO3 [14], Ca-Sn [15] LDHs have been manufactured by the solvent-free mechanochemical process.In this work, we reported the fundamental data obtained with Li-Al LDH synthesized via a one-step mechanochemical route.Meanwhile, the synthetic Li-Al LDH was added into boron solution to understand their adsorption performance.Furthermore, to elucidate the reaction mechanism and the adsorption mechanism involved, we performed physics measurements and equilibrium studies on the boron up taking process.

Preparation of LDH
Two-gram mixture of LiOH and Al(OH)3 was milled at alterable Li/Al molar ratio to prepare Li-Al LDH in a planet mill (QM-3SP04, Nanjing NanDa Instrument Plant, China), which has four mill pots (50 cm 3 inner volume each) made of stainless-steel with 7 steel-balls of 17 mm diameter.Mill speed in this work was kept constant at 500 rpm (auto rotational speed).The milling time was set to be 2 h, 3 h, 4 h, 5 h, respectively and the Li/Al molar ratio was determined to be 1/2, 2/2.No washing or other treatment was done with the powder before X-ray diffraction, FTIR analysis, and surface zeta potential test.

Adsorption experiments
All solutions were prepared and stocked in polyethylene block container.Boric acid was Aquamate 8000, Thermo, America) at 420 nm.The powder in the bottom of the bottles was collected and dried for 2 h at 50 ℃ for X-ray diffraction, FTIR analysis, and surface potential test.

Physics measurements
Powder X-ray diffraction patterns of the samples were recorded on a Rigaku MAX-RB RU-200B diffractometer using CuKa radiation (λ= 1.5403 Å) at the scanning rate of 15° min -1 and step size of 0.02° in the 2θ range of 3°-70°, operating at 20 kV and 50 mA.Fourier transformed infrared (FT-IR) (Nicolet6700, Thermo, America) spectra of the samples were measured using KBr as a diluent over 4000-500 cm -1 .Firstly, samples were mixed with KBr in a 1/200 ratio of their mass and then pressed to form a pellet for the measurement by diffuse reflection-Flourier transformed infrared spectroscopy from 400 cm -1 to 4000 cm -1 .The zeta potential of the powder was analyzed by zeta meter (Malvern, Zetasizer90, UK).
3 Results and discussion Al(OH)3 to form Li-Al LDH was confirmed and the corresponding adsorption capacity of the sample reached 28.21 mg/g.Longer time milling than 3 h weakened the degree of crystallinity and the corresponding adsorption capacity of the sample obviously declined, which indicated 3 hours may be the optimum point.

Effect of molar ratio on adsorption capacity
By keeping 3 h of milling, the effect of molar ratio of Li/Al on the preparation of LDH was investigated by comparing two mixtures with the ratio of 1/2 and 2/2, respectively and the results were presented in Fig. 3.Although LDH phase was observed from both two patterns, only the ratio of Li/Al at 1/2 gave a pure phase without observable impurity phases.
In the upper pattern, namely the starting materials LiOH/Al(OH)3 at 2/2, peaks of LiOH•H2O and LiAlO2 were observed.Starting material LiOH remained in the products as LiOH•H2O due to its excessive addition.The formation of LiAlO2 in the milled mixtures was due to the solid reaction between Al(OH)3 and LiOH [18].
The high value of Li/Al does no help to the formation of LDH structure for chemical reaction between Al(OH)3 and redundant LiOH as shown in Fig. 3 (Eq.2-4).
Total reaction progress can be presented as Eq.5:

Adsorption mechanism
Generally, the boron ion can be removed by LDH through two mechanisms: ion-exchange (indicated by the increasing of d003 spacing [17]) and surface adsorption (indicated by the shifting of zeta potential to negative value [20,21]).In order to identify the adsorption mechanism, XRD patterns were carried out as shown in Fig. 5.The interlayer space d003 was 2.47 Å before adsorption and 2.49 Å after adsorption.Nearly invariable d003 indicated that the mechanism of boron uptake for the LDH was not ion-exchange and interlayer -OH made no difference to remove boron.Miyata [22] proved -OH possesses the highest position of the ion selectivity of HT-like compounds which cause none ion-exchange between boron species and -OH.The zeta potential of the materials before and after adsorption shifted to negative direction from -1.85 mV to -11.15 mV in pH 8.75 which coincided well with experimental results from Kentjono et al. [20] implying the adsorption of boron on the external surface of the material by surface complex formation.
Fig. 5 shows the FT-IR patterns of the sample (B) before adsorption and sample (A) after adsorption.Infrared bands positioned 3465 cm −1 , 1649 cm −1 , 1637 cm −1 , 1027 cm −1 and 534 cm −1 observed in the spectrum of sample (B) were attributed to Li-Al LDH [10].The bands positioned at 866 cm −1 observed from the spectrum of the sample was attributed to lithium hydroxide monohydrate [10].A doublet at 1437 and 1503 cm -1 in (B) may be the CO3 absorption band [18] confirming CO2 adsorption from ambient because milling was conducted in the atmosphere and LiOH would easily react with CO2 [10].Infrared bands around 788 cm −1 and 696 cm −1 were attributed to the LiAlO2.
In comparison, the spectrum of the hydroxide after adsorption represents the typical bands of Li-Al LDH sample from the previous reports [10].The spectrum of sample (A) has a broader absorption band in the -OH stretching region centering at 3471 cm -1 .The LiAlO2 in the sample transformed to phase gel Al(OH)3 as shown in Eq.5 and helped to adsorb boron acid.Zhu et al. [23] proved phase gel Al(OH)3 can react with H3BO3 as shown in Eq. 6.The absence of any sharp peaks in this region at around 3471 cm -1 proved the chemical analyses that boron was held directly to the hydroxide surface.The infrared bands around 1016 cm −1 and 944 cm −1 observed from the spectrum of the sample (A) were attributed to tetrahedral borate [24] and B(OH)4 - [16], respectively [10], confirming the presence of B(OH)4 -units on the surface of the sample.respectively.Maximum adsorption capacity of boron removal from optoelectronic wastewater using Mg-Al (NO3) LDH was reported to be 37.9 mg/g [20].Boron uptake capacity of Li-Al LDH in our experiment was comparable with previous reports.

Conclusions
Li-Al-OH LDH was prepared by solvent-free one-step milling and was used to adsorb boron from aqueous solution in this study.The experimental results suggested the following

Preprints
(www.preprints.org)| NOT PEER-REVIEWED | Posted: 20 November 2018 Preprints (www.preprints.org)| NOT PEER-REVIEWED | Posted: 20 November 2018 Preprints (www.preprints.org)| NOT PEER-REVIEWED | Posted: 20 November 2018 doi:10.20944/preprints201811.0477.v1used to prepare B solutions for batch adsorption experiments and B quantitative analysis.A certain amount of boric acid was added into 500 ml capped polyethylene bottles and dissolved in 100 ml deionized water, then 0.1 g of LDH sample was added into the B solution, following by shaking for 5 h with water bath (DKZ-2, Yiheng Shanghai, China) at approximately 25 ℃.Finally, the obtained solutions were centrifuged (LXJ, Anke Shanghai, China).The supernatant liquor was analyzed residual boron concentration using azomethine-H hydrates color-developing agent in UV-VIS spectrophotometer (Orion

3. 1
Fig.1shows the X-ray diffraction patterns for the as-prepared LDH at different milling

Fig. 4 (
Fig.4 (a) shows batch adsorption results of the prepared Li-Al LDH products dry-milled

Fig. 1
Fig. 1 XRD patterns of Li-Al LDH sample prepared at different milling times.

Fig. 2
Fig. 2 Boron adsorption by prepared LDHs as a function of milling times without pH adjustment.

Fig. 6 FT
Fig. 6 FT-IR patterns of Li-Al-OH LDH studied before adsorption (B) and after adsorption (A).

Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 20 November 2018 Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 20 November 2018 Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 20 November 2018 doi:10.20944/preprints201811.0477.v1
Three hours of milling at 500 rpm, Li/Al molar ratio 2/2 was the optimum condition to prepare superior B adsorption capacity of Li-Al-OH LDH.Adsorption isotherms suggest maximum monolayer boron uptake capacity can reach 45.45 mg/g which is competitive to other LDH boron adsorbents.2.XRD patterns of the sample before and after adsorption shows little increasing of d003spacing indicating the ion-exchange between boron species and OH -does limited help to remove boron.FT-IR patterns of the sample after adsorption confirmed the vibration of anion boron species.Zeta potential shifting to negative direction illustrated boron adsorption on the surface of the material.
3. Phase gel Al(OH)3 came from hydrolysis of LiAlO2 may do considerable contributions to boron removal by chemical absorption.