XML : Adding namespace in XML after dynamically populating prefixed Attribute in Scala

I am new to Scala and stuck on this issue. I am trying to construct a SOAP XML message using scala.xml. I need to dynamically add the nil attribute to the nodes that have no value. I am doing that as below

   <temp>{tag._2}</temp>.copy(label=tag._1)% Attribute("xsi","nil",Text("true"),Null)    

This generates the string I need but on the SOAP envelope it also adds the prefixed namespace as xmlns:xsi="xsi" shown below

      <SOAP-ENV:Envelope .... xmlns:xsi="xsi">      ...      <employee>       <firstname>John</firstname>       <lastname>Smith</lastname>       <dob xsi:nil="true"/>      </employee>      <employee>       ....      </employee>      </SOAP-ENV:Envelope>    

I need the namespace value to be something else. Is there a way to specify the namespace for this prefixed attribute?