Aerogel: Cross-Linking!

Week #06: Adsorption Optimization, Ion Exchange, Chelation

Carlos Manuel Jarquín Sánchez
4 min readSep 9, 2022


Preface ✨

Hello everyone!

My intentions for writing these articles are:

  • Explain technical information about aerogels in simple terms (to the public)
  • Store information and habits for my future self and others (in <7 minutes)

Coolio? Sweet. Enjoy the series :-)

What Is Cross-Linking? 💭

Cross-Linking is the essential procedure to convert sodium alginate (alginic acid) into a functional 3-D material: A heavy metal ion water filter.

Cross-linking can change the solubility level of sodium alginate. This allows the aerogel filter to increase in mechanical strength. It makes the aerogel filter flexible, strong, and adsorb heavy metal ions… like a sponge.


But cross-linking eliminates sorption sites on the sodium alginate when they react with the adsorption groups (-NH2, -COOH, -OH).

You should choose to measure the adequate dosage of effective and affordable cross-linkers to optimize filter performance and regeneration.

But Carlos! Why do we need to do this cross-linking nonsense? Do we have to?

You do not necessarily have to perform cross-linking on sodium alginate-based aerogels. But no cross-linking or the wrong cross-linkers results in weaker mechanical strength and lack of reusability.

The following photos are before & after shots of certain cross-linkers to make aerogel filters.

No Cross-Linking Present [MEK]
Wrong Cross-Linker [Acetone]
Effective Cross-Linker [Methanol, Hydrophobic]

The necessary criteria for an effective cross-linker are:

  1. Have five or more “bonding atoms” to attach to the heavy metal ion
  2. Contain functional groups (metal-adsorbing molecules)
  3. Adsorb heavy metal ions via ion exchange and/or chelation
  4. Anionic (negatively charged) & a pH lower than 7

Ion Exchange Versus Chelation: Who Wins? ⚛️

Ion Exchange is a process where heavy metal ions are replaced from the water by other ions that do not contribute to humanitarian or environmental damage. [Some safe ions include Calcium Ions (Ca2+) or Hydrogen Ions (H+)]

Chelation is a process that bonds the functional groups and metal ions in a specific arrangement. This arrangement requires three or more atoms attaching to the central heavy metal ion.

Ion Exchange Replaces One Ion With Another | Chelation Rearranges The Attachment Of Ion To The Filter

A fluid that can create stronger bonds with heavy metal ions than the filter is necessary to regenerate the filter for multiple adsorptions. The fluid/chemical must have a stronger bond than carboxyl groups [COOH], hydroxyl groups [OH], and amines [-NH, NH2].

The determining factor is the pKa.

The lower the pKa number is, the measured acid can be very strong and have a higher chance of “giving away” its protons. Giving away its protons results in a stronger attraction to heavy metal ions. The strongest acids will have a pKa less than zero.

The most effective cross-linker for the ion-exchange & chelation method is Calcium Chloride [CaCl2], (Ca2+). It works effectively because Sodium Alginate also has cationic ions (Na+). The gel structure increases in mechanical strength when the ions from CaCl2 & Na+ combine.

Divalent heavy metal ions [Pb2+ (Lead), Cd2+ (Cadmium), Hg2+ (Mercury), etc.] have a preference to bond with CaCl2 than any other heavy metal ions [(monovalent; Li+ Cs+) (trivalent; Cr3+, Fe3+, Al3+)].

But why, Carlos?

The ionic radius for CaCl2 (Ca2+ ions specifically) is similar to the ionic radius of divalent heavy metal ions, including Na+ from sodium alginate. This creates a stronger ion-exchange effect to remove heavy metal ions from water as long as the radius of the metal ion is close to the radius of the Ca2+ ion.

Ca2+ ionic radius is 0.100 nanometers. Pb2+ ionic radius is 0.119 nanometers. Cd2+ ionic radius is 0.095 nanometers. Hg2+ ionic radius is 0.102 nanometers. Na+ ionic radius is 0.102 nanometers.

Ex: Monovalent Lithium Ion (Li+) ionic radius is 0.076 nanometers.

Ex: Trivalent Iron Ion (Fe3+) ionic radius is 0.060 nanometers.

The Molecular Chelation With Sodium Alginate & Calcium Chloride Cross-Linker

And What Are The Limitations? 🛑

The main caveat for ion exchange: The heavy metal ion must be in the proper ionic form (divalent) and must bind more strongly than the safe, displaced ion (ex: Ca2+ or H+)

The selection of the right cross-linker is the best way to mitigate a water filter catastrophe.

The main caveat for chelation: The pH level must be between a pH of 4 to a pH of 6 because the optimized adsorption rates are located in this range. Cationic heavy metal ions will be adsorbed more than anionic heavy metal ions within a pH of 4 - 6.

The majority of wastewater sources in the world fall in a pH range of 5. There is no need for adjusting pH levels within the filter.

The best options for a new cross-linker to the filter: Ethylenediamine (EN) & Diethylenediamine (DETA).

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© 2023 by Carlos Manuel Jarquín Sánchez. All Rights Reserved.