Start-Process in a for loop Powershell

Hi,

I am trying to write a script that starts a defrag on selected volumes. The idea is to get the volume letters of volumes that are greater than 50GB, loop through each letter and add it to a line that calls defrag.exe /H /U /V, and waits until each defrag instance has exited before starting the next. 

Here are my lines:

#Lets do some defrag
$Defrag = 'C:\Windows\System32\Defrag.exe'
$Argumentals = '/H /U /V'
#Get volumes
$MinVolSize = 53687091200 #50GB
$Volumes = @((Get-WmiObject -Class Win32_LogicalDisk) | 
           ? {$_.Size -gt $MinVolSize}).DeviceID

for($vol=0;$vol -lt $Volumes.Length;$vol++){
    
         Write-Host $Defrag $($Volumes[$vol]) $Argumentals
      
}

When I run this, I get the following output in the console:

C:\Windows\System32\Defrag.exe C: /H /U /V
C:\Windows\System32\Defrag.exe D: /H /U /V
C:\Windows\System32\Defrag.exe E: /H /U /V
C:\Windows\System32\Defrag.exe F: /H /U /V

But then if I change the "Write-Host" to "Start-Process", I get 4 instances of this:

Start-Process : A positional parameter cannot be found that accepts argument '/H /U /V'.
At C:\Users\a-ahinton\SkyDrive\Scripts\Overnight.Maintainence.ps1:34 char:10
+          Start-Process $Defrag $($Volumes[$vol]) $Argumentals
+          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidArgument: (:) [Start-Process], ParameterBindingException
    + FullyQualifiedErrorId : PositionalParameterNotFound,Microsoft.PowerShell.Commands.StartProcessCommand

Would someone please point me in the direction of information that will teach me why this is happening and what I need to know to fix it or prevent this error from throwing? 

Any links, key terms and criticism will be received with much appreciation and many thanks in advance.