Sulfate Separation by Selective Crystallization with a Bis-iminoguanidinium Ligand

Summary

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

Abstract

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 β 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.

Introduction

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 crysta....

Protocol

1. Synthesis of 1,4-Benzene-bis(iminoguanidinium) Chloride (BBIG-Cl)

1. In Situ Synthesis of the 1,4-Benzene-bis(iminoguanidinium) Chloride Ligand (BBIG-Cl) and Its Crystallization with Sulfate
   1. 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.
   2. Stir the solution magnetically for four hours at 20 °C. This will yield a slightly yellow solution of BBIG-Cl.
   3. Add 0.5 ml of a 1 M aqueous solution of sodium sulfate. This will result in the instant precipitation of BBIG-SO₄....

Results

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.

Discussion

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 β liquid scintillation counting section need to be f.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Terephthalaldehyde	Sigma	T2207
Aminoguanidinium Chloride	Sigma	#396494
Sodium Sulfate	Sigma	#239313
Barium Chloride	Sigma	#342920	Highly Toxic
Ethanol	Any		Reagent Grade (190 proof)
Sodium Hydroxide	EMD	SX0590-1
Hydrochloric Acid	Sigma	#258148
Filter Paper	Any	-	Any qualitative or analytical filter paper will work
Syringe Filter (0.22 μm)	Any	-	Nylon filter
35S Labeled Sulfate	Perkin Elmer	NEX041005MC
Ultima Gold Scintillation Cocktail	Perkin Elmer	#6013329
Polypropylene Vials	Any	-
Disposable Syringe (2-3 ml)	Any	-	Any disposable plastic syringe works