This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. We thank the University of North Carolina Wilmington for providing the seawater.

1. Synthesis of 1,4-Benzene-bis(iminoguanidinium) Chloride (BBIG-Cl)
<ol>
	<li>In Situ Synthesis of the 1,4-Benzene-bis(iminoguanidinium) Chloride Ligand (BBIG-Cl) and Its Crystallization with Sulfate
	<ol>
		<li>Add 0.067 g of terephthalaldehyde and 2.2 ml of a 0.5 M aqueous solution of aminoguanidinium chloride to 10 ml of deionized water in a 25 ml round bottom flask equipped with a magnetic stir bar.</li>
		<li>Stir the solution magnetically for four hours at 20 &#176;C. This will yield a slightly yellow s...

<ol>
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This technique is rather tolerant to many deviations from the written procedure, which makes it quite robust. There are however two critical steps that must be followed. First, the BBIG-Cl ligand needs to be as pure as possible. Impurities will not only affect the crystallization and the solubility of the resulting sulfate salt, but will also make it difficult to calculate the amount required for quantitative sulfate removal from solution. Second, all steps in the &#946; liquid scintillation counting section need to be f...

Selective separation of hydrophilic oxoanions (e.g., sulfate, chromate, phosphate) from competitive aqueous solutions represents a fundamental challenge with relevance to environmental remediation, energy production, and human health.1,2 Sulfate in particular is difficult to extract from water due to its intrinsic reluctance to shed its hydration sphere and migrate into less polar environments.3 Making aqueous sulfate extraction more efficient typically requires complex receptors that are difficult and tedious to synthesize and purify, often involving toxic reagents and solvents.4,5
Selective crystallization offers a simple yet effective alternative to sulfate separation from water.6-9 Though some metal cations such as Ba2+, Pb2+, or Ra2+ form very insoluble sulfate salts, their use in sulfate separation is not always practical due to their high toxicity and sometimes-low selectivity. Employing organic ligands as sulfate precipitants takes advantage of the structural diversity and amenability to design characteristic to organic molecules. An ideal organic ligand for aqueous sulfate crystallization should be soluble in water, yet form an insoluble sulfate salt or complex in a relatively short time and in the presence of high concentrations of competing ions. Additionally, it should be easy to synthesize and recycle. One such a ligand, 1,4-benzene-bis(iminoguanidinium) (BBIG), self-assembled in situ from two commercially available precursors, terephthalaldehyde and aminoguanidinium chloride, was recently found to be extremely effective in aqueous sulfate separation.10 The ligand is water-soluble in the chloride form, and selectively crystallizes with sulfate into an extremely insoluble salt that can be easily removed from solution by simple filtration. The BBIG ligand can then be recovered by deprotonation with aqueous NaOH and crystallization of the neutral bis-iminoguanidine, which can be converted back into the chloride form with aqueous HCl, and reused in another separation cycle. The efficacy of this ligand in removing sulfate from water is so great that monitoring the remaining sulfate concentration in solution is no longer a trivial task, requiring a more advanced technique that allows accurate measurement of trace amounts of the anion. For this purpose, radiolabeled 35S sulfate tracer in conjunction with &#946; liquid scintillation counting was employed, a technique commonly utilized in liquid-liquid extractive separations, and recently demonstrated to be effective in monitoring sulfate crystallization.8
This protocol demonstrates the one-pot in situ synthesis of the BBIG ligand and its crystallization as the sulfate salt from aqueous solutions. The ex situ synthesis of the ligand11 is also presented as a convenient method for the production of larger amounts of BBIG-Cl, which can be stored in the crystalline form until ready to use. Sulfate removal from seawater using the previously prepared BBIG-Cl ligand is then demonstrated. Finally, the use of 35S-labeled sulfate and &#946; liquid scintillation counting for measuring the sulfate concentration in seawater is demonstrated. This protocol is intended to provide a tutorial for those broadly interested in exploring the use of selective crystallization for aqueous anion separation.

<table border="0" cellpadding="0" cellspacing="0" width="740">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Terephthalaldehyde</td>
			<td style="width:92px;">Sigma</td>
			<td style="width:119px;">T2207</td>
			<td style="width:313px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Aminoguanidinium Chloride</td>
			<td style="width:92px;">Sigma</td>
			<td style="width:119px;">#396494</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Sodium Sulfate</td>
			<td style="width:92px;">Sigma</td>
			<td style="width:119px;">#239313</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Barium Chloride</td>
			<td style="width:92px;">Sigma</td>
			<td style="width:119px;">#342920</td>
			<td>Highly Toxic</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Ethanol</td>
			<td style="width:92px;">Any</td>
			<td style="width:119px;"></td>
			<td>Reagent Grade (190 proof)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Sodium Hydroxide</td>
			<td style="width:92px;">EMD</td>
			<td style="width:119px;">SX0590-1</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Hydrochloric Acid</td>
			<td style="width:92px;">Sigma</td>
			<td style="width:119px;">#258148</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Filter Paper</td>
			<td style="width:92px;">Any</td>
			<td style="width:119px;">-</td>
			<td>Any qualitative or analytical filter paper will work</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Syringe Filter (0.22 um)</td>
			<td style="width:92px;">Any</td>
			<td style="width:119px;">-</td>
			<td>Nylon filter</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">35S Labeled Sulfate</td>
			<td style="width:92px;">Perkin Elmer</td>
			<td style="width:119px;">NEX041005MC</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Ultima Gold Scintillation Cocktail</td>
			<td style="width:92px;">Perkin Elmer</td>
			<td style="width:119px;">#6013329</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Polypropylene Vials&#160;</td>
			<td style="width:92px;">Any</td>
			<td style="width:119px;">-</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Disposable Syringe (2-3 mL)</td>
			<td style="width:92px;">Any</td>
			<td style="width:119px;">-</td>
			<td>Any disposable plastic syringe works</td>
		</tr>
	</tbody>
</table>

sulfate separation by selective crystallization with a bis-iminoguanidinium ligand

A simple and effective method for selective sulfate separation from aqueous solutions by crystallization with a bis-guanidinium ligand, 1,4-benzene-bis(iminoguanidinium) (BBIG), is demonstrated. The ligand is synthesized as the chloride salt (BBIG-Cl) by in situ imine condensation of terephthalaldehyde with aminoguanidinium chloride in water, followed by crystallization as the sulfate salt (BBIG-SO4). Alternatively, BBIG-Cl is synthesized ex situ in larger scale from ethanol. The sulfate separation ability of the BBIG ligand is demonstrated by selective and quantitative crystallization of sulfate from seawater. The ligand can be recycled by neutralization of BBIG-SO4 with aqueous NaOH and crystallization of the neutral bis-iminoguanidine, which can be converted back into BBIG-Cl with aqueous HCl and reused in another separation cycle. Finally, 35S-labeled sulfate and &#946; liquid scintillation counting are employed for monitoring the sulfate concentration in solution. Overall, this protocol will instruct the user in the necessary skills to synthesize a ligand, employ it in the selective crystallization of sulfate from aqueous solutions, and quantify the separation efficiency.

The powder X-ray diffraction pattern of BBIG-SO4 (Figure 1) allows for unambiguous confirmation of the identity of the crystallized solid. In comparing the obtained pattern versus the reference one, peak intensity matters less than peak positioning. All strong peaks shown in the reference should be present in the obtained sample. The appearance of strong peaks in the sample that are absent in the reference pattern indicates the presence of impurities.
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Sulfate Separation by Selective Crystallization with a Bis-iminoguanidinium Ligand

A protocol for in situ aqueous synthesis of a bis(iminoguanidinium) ligand and its utilization in selective separation of sulfate is presented.

A simple and effective method for selective sulfate separation from aqueous solutions by crystallization with a bis-guanidinium ligand, ...

The overall goal of this procedure is to separate the sulfate anions from competitive aqueous solutions by selective crystallization with a bis-guanidinium ligand synthesized in a single step from simple precursors.Sulfate separation from competitive aqueous solutions is challenging due to the strong hydrophilic nature of this anion and typically requires complex receptors that are difficult to synthesize and purify.The main advantage of this technique is that it combines the in situ ligand synthesis and anion separation into a single step.To begin the in situ procedure, add 10 milliliters of deionized water to a round-bottomed flask equipped with a magnetic stir bar.Then add 067 grams of terephthalaldehyde and 2.2 milliliters of a 0.5 molar aqueous solution of aminoguanidinium chloride.Stir the solution for four hours at 20 degrees Celsius.This will yield a slightly yellow solution of BIG chloride.Once the stirring is complete, add 0.5 milliliters of a one molar aqueous solution of sodium sulfate.This will precipitate out 1, 4 benzene bis aminoguanidinium sulfate as a crystalline white solid.Next, utilize vacuum filtration to recover the BIG sulfate.Wash the recovered solid on the filter paper five times with five milliliter aliquots of water to obtain the pure sulfate salt.Using powder x-ray defraction, check the phase purity of the crystalline BIG sulfate as described.For ex situ synthesis, first set up a 50 milliliter round-bottomed flash with a stir bar and hot plate.Add 20 milliliters of ethanol.Next, add four grams of terepthalaldehyde and 7.26 grams of aminoguanidinium chloride.Heat the solution to 60 degrees Celsius and stir it for two hours.Then cool the solution to 20 degrees Celsius and let it rest for three hours.Utilize a filter paper-equipped B

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8.3K vistas.  Oak Ridge National Laboratory. The overall goal of this procedure is to separate the sulfate anions from competitive aqueous solutions by selective crystallization with a bis-guanidinium ligand synthesized in a single step from simple precursors.Sulfate separation from competitive aqueous solutions is challenging due to the strong hydrophilic nature of this anion and typically requires complex receptors that are difficult to synthesize and purify.The main advantage of this technique is that it combines the in situ ligand s...