Dear غزوان حسن الصميدعي, ليس كل المشتركين له القدرة على قراءة العربية , not all people on RG are able to read Arabic.
First I translate the answer of Dr. Ghazwan: any washing procédure is limited by a certain level, which defines the number of repeated times of washing, and following that by measuring the amount of salt.
My answer : if your Ferrihydrite are nanoparticles, I am afraid you can not find a dialysis tube that goes to such low cutoff. Repeated washing is efficient but there is risk of loosing some of the product besides the integrety of you particles that may be affected. My Regards