Dear Alice,

Frankly, I do not think that membranes with a MWCO of 1000 Dalton can effectively reject sulfates.

The following are the membranes used for the removal of sulfate ions:

GE’s Seawater Sulfate Reducing Membrane (SWSR) is an NF membrane developed for seawater sulfate removal. It is built on GE's robust DK membrane platform, which has been used for over 30 years in some of the most challenging

applications. The SWSR Nanofiltration membrane features a 300 Dalton Molecular Weight cut-off and is an absolute barrier for any suspended matter including particles, colloidal silica, and bacterial matters. Additional benefits include:

 Prevents strontium and barium sulfate scale in injection wells

 Better well souring mitigation by reducing the incoming sulfate

 Optimized for use in seawater while providing superior hydrodynamics, which result in longer membrane life

 Unique and long-proven GE 3-layer membrane structure that minimizes surface roughness and adherence

 Available in a 440 sq. ft. version to maximize membrane area for new systems while minimizing footprint

 Superior hardness and sulfate removal properties  Chemical compatibility with cleaners currently used in the industry

 Innovative spacer design combined with 3-layer membrane allows improved fouling resistance

 Can be combined with RO membrane arrays in order to provide low TDS water with specific divalent ion concentrations

For more on this topic, please see attached file.

Hoping this will be helpful,

Rafik

Usually, MWCO values are calculated from the linear polymers (PEG). On the other hand, hydrated sulfate ions are bulky (>10 nm diameter). That's why, 1000 Da membrane can be used for sulfate removal.

Hi Alice,

According to the following course notes (page 23) you will need a 5-100nm UF membrane.

http://www.apexengineering.us/Presentation_4_--_Established_Treatment_Technologies_Benowitz.pdf

Hope this is helpful.

Kind Regards,

Haitham

Hello again,

Also check Table 2 of this publication,

http://www.drink-water-eng-sci.net/6/47/2013/dwes-6-47-2013.pdf

I think its better to use a nano-filtration membrane instead of a tight ultra-filtration membrane.

Cheers,

Haitham

Nanofiltration (NF) is a pressure-driven membrane process with performance characteristics between reverse osmosis (RO) and ultrafiltration. The theoretical pore size of the membrane is 1 nm, although there are no pores in thin-film composite spiral-wound NF membranes (when made of polyamide the surface is slightly negative).NF will have a high removal (high rejection) of sulfate because it is a divalent ion, but will have lower rejection of monovalent ions such as sodium. 

Mass transfer of solutes across the active membrane thin-film can be predicted using the homogeneous solution-diffusion model as first illustrated by Kedem and Katchalsky in 1958 (Biochimica et Biophyica Acta, 27:123) as well as with the use of sieving models. I would agree with others that a molecular weight cutoff of between 300 and 400 would suffice for sulfate removal using a NF membrane process, as sulfate removal is primarily a function of the Donnan effect. I would also agree that you would not see much sulfate removal in a membrane having a MW cutoff of 1000 or more.

Separation of ionic species by NF strongly depends on the membrane charge and molecular weight cutoff. A membrane with a smaller MW cutoff is better able to retain ionic species. Likewise, a highly charged membrane is better able to exclude co-ions (ions of same charge as the membrane) from the membrane surface. RO for sulfate removal is generally only considered when monovalent contaminants must also be removed.  Otherwise, NF is more cost-effective for sulfate removal than is RO.

See:

Peeters et al. (1998). Retention measurements of nanofiltration membranes with electrolyte solutions. J. Membr. Sci. 145, 199.

Tsuru et al. (1991). Reverse osmosis of single and mixed electrolytes with charged membranes: Experiments and analysis. J.Chem. Eng. Jpn. 24, 518.

Duranceau and Taylor. "Solute Charge and Molecule Weight Modeling for Prediction of Solute Mass Transfer Coefficients." American Water Works Association Seminar Proceedings: 1993 Membrane Technology Conference. Baltimore, MD: AWWA, 243-249 (1993).

Thank you, everyone!

I got a pretty good idea from your answers.

All of them were really useful!

Actually, I think the key point is that there is no standard method for the determination of MWCO of membrane. If you could specify how does the MWCO of 1000 Da come, the answer would be clear. Let's say, it is determined by the PEG filtration method. PEG is a linear and neutral chemical, and its molecular structure is also quite flexible. So even its MW is 1000 Da, it could still pass through the membrane. But how about other chemicals, such as PPCP or benzene？the answer might be different. The structure of membrane selective layer is also another aspect that should be considered. According to what I know, membrane's selective layer is some like 3 dimensions net-work of polymers chains. It has no pores but the free volume. hope this could answer your question.