SAMMS® stands for “Self-Assembled Monolayers on Mesoporous Supports.” It describes a family of “designer” adsorbents made using molecular-engineering. A mesoporous material is one that has many pores about the size of tens to hundreds of atoms wide. If these pores were connected end to end approximately 100,000 would span the distance of a single human hair. A surface that holds such pores has within 1 gram (0.002 lb) of material a total surface area as large as a football field. There is great power in an adsorbent whose every surface atom can bind a desired element such as mercury.

What is Self Assembly?

“Self-assembly” of SAMMS involves using silane chemistry. Silane compounds are a highly developed class of materials whose chemical structure is virtually the same as organic materials except the carbon atoms of the organic compound are replaced by silicon atoms. Silanes can merge both carbon chemistry and silicon chemistry to design very unique chemicals.

In the left hand part of figure 1 the silane is the string that has a yellow ball (molecule) and a red ball (molecule) on each end. We call the silane the “tether” because it is going to tether the active adsorbent to the surface of an adsorbent support. The red ball is a hydroxyl group (part of a water molecule). It is part of the normal silane. The yellow ball is the adsorbent. It is a specific chemical compound selected to adsorb a metal that is attached to the silane using chemical methods. In the present case the yellow ball is a thiol group. A thiol is a compound that is highly reactive with mercury. When mercury is present it creates a very strong chemical bond to the thiol group. Self assembly is a special chemical process that attaches the silane carrying the thiol to the surface that we call the “adsorbent support.” The attachment process is called self-assembly because it attaches silane to the surface in a highly ordered way. In normal chemical reactions the silanes will attach as single tethers and are “floppy.” This is illustrated by the center image. The right image is the ordered self-assembly.

Figure 1: An illustration of self assembly of silane tethers to a surface. Rater than forming a random unorganized attachment of silane resembling a fuzzy ball, self-assembly causes the silanes to attach in an orderly manner with all the thiol groups facing outward. Self assembly is actually nature’s method to build many types of living cells and materials.

Figure 2: A high magnification illustration of attaching silane tethers to the hydroxide on the surface of a ferrite support so there is a one-for-one thiol group to each hydroxide. Even more complex silanes can be made and attached that have 2 or 3 thiols per tether. This multiplies the effect of adsorbent.

Figure 3, illustrates the surface and pore geometry of Thiol-SAMMS. Figure 4, is a higher magnification schematic of the inside of the pore of Figure 3 on a microscopic scale. This figure illustrates a pore on the high surface area base that has the thiol groups attached to the inner surface of its pores. The thiol (yellow) groups are tethered to the pore surface by the silane as previously described. As long as the pores are appreciably bigger than the mercury atom (blue) to diffuse into, the adsorbent will adsorb mercury by a chemical bonding reaction to remove mercury that is extremely fast. Figure 5, illustrates why Thiol-SAMMS is such a powerful adsorbent. Since the surface area of SAMMS is very large, the material can adsorb a relatively large quantity of mercury compared to its own weight.

Atomic Level Engineering

Figure 2 is a second illustration of self assembly used by Steward to attach a specific silane compound holding a thiol group to a ferrite surface. Self assembly puts a silane on each hydroxide on the surface of the support.

Why a Mesoporous Material?

The surface area of ordinary silica is about 1 square meter per gram. It is too low to make a good adsorbent because there are not enough silicon atoms on the surface. If we make pores, or holes in the surface of a silica particle we can increase the total exposed surface of silicon atoms. The best analogy would be to take a golf ball and drill 1/32 inch wide, 1/4 inch-deep holes all over its surface so each drill hole almost touches the next. Now if we add up all the surface area inside the drill holes of the golf ball we have greatly magnified the total surface area of the formerly dimpled, hard-surfaced ball. Figure 3 is a schematic representation of pores in a zeolite that have been functionalized by SAMMS technology.

Engineered mesoporous silica can have a surface area up to 500 to 1000 m2 per gram. With such surface area there are enough atoms to attach thiol molecules that we can bind mercury up to 60% or more of the weight of the silica. This gives SAMMS adsorbent the highest known capacity to adsorb mercury.

Significant Benefits of SAMMS:

Kinetics – SAMMS has extremely fast kinetics and tests have demonstrated mercury reductions to below 1 part per trillion. Absolute mercury reduction is dependent on the chemical conditions of the solution. Delivery systems are available in both batch and high flow rates configurations for systems typically encountered in process water applications. Optimal kinetics is typically achieved from 4 to 8 pH and good results are obtained from a pH of ~3 to 12.

Capacity – The typical capacity or loading factor of SAMMS derived from the adsorption isotherm is 0.4 – 0.6 grams Hg/gram of the Thiol-SAMMS adsorbent for terminal Hg concentration of 100-200 ppm. This makes SAMMS the only commercially available adsorbent that can absorb more than half its weight in mercury.

Selectivity – Most cations and anions have a minimal impact on SAMMS performance and SAMMS is highly selective for mercury and other “soft” heavy metals such as silver, cadmium, copper and lead. SAMMS is chemically specific and hydrophobic which are characteristic that enhance its ability to remove mercury in the presence of organics in aqueous waste steams.

Adsorption Capabilities – SAMMS posseses superior adsorption capabilities by covalently bonding to remove mercury from contaminated groundwater and liquid waste streams.

Cost-Effective- SAMMS is a cost effective alternative to other commercially available sorbents when comparing the full cost benefits. Compared to other sorbents SAMMS is able to adsorb large quantities of mercury with little solid waste generated for disposal. In addition, SAAMS can be regenerated to recover the mercury and reuse the adsorbent to further lower operational cost.

Material Properties:

• Particle Size – The average particle size for SAMMS is approximately 40 micron.
• Bulk Density – The bulk density for SAMMS is approximately 0.3 g/cc.

Regeneration and Stripping:

It is possible to strip the mercury from the Thiol-SAMMS using a strong acid after adsorption. The SAMMS can be reused and the mercury recovered. Leachability and Disposal: The binding strength of thiol ligands with mercury is very strong and in most cases allows SAMMS to pass TCLP requirements. However, users must perform a leachate test to verify that all local, state and federal laws and regulations for disposal are met.