(Reuters) - Nelnet <NNI.N> said it bought a residual interest in $1.9 billion of federal student loan from an affiliate of Greystone & Co Inc.

The student finance company acquired the interest in notes issued by GCO Student Loan Interest Margin Securities Trust-I (GCO SLIMS Trust-1), giving the company rights to the residual interest in GCO Education Loan Funding Trust-I.

The company expects the purchase to add immediately to base net income.

Nelnet said the loans and debt within the trusts will increase its student loan portfolio to more than $25 billion.

(Reporting by Anil D'Silva; Editing by Don Sebastian)