Laravel - Unable to pass variable value for mysql column name

Herewith I need to pass a variable value instead of column name of mysql table. At the beginning I got that variable value from select query return as

public function getSiteGrading($site)
    {                    
        $siteGrading = DB::table('sites')
                    ->select('siteGrading')
                    ->where('code',$site)                    
                    ->get(); 

        return $siteGrading;                 
    }

Then I need to use that return variable value '$siteG' as a column name for another select query as

public function getItemStatus($itemCode)
    {
        $ordSite = Auth::user()->name;
        $siteG = $this->getSiteGrading($ordSite);
        // dd($siteG);

        $mandatory_count = DB::table('item_statuses')
                    ->select('itemCode')
                    ->where('itemCode',$itemCode)
                    ->where($siteG,'M')
                    ->get(); 
                    // dd($mandatory_count);

        return $mandatory_count;                 
    }

Finally from the 'getItemStatus' function, I got below error saying wrong column name. 'SQLSTATE[42S22]: Column not found: 1054 Unknown column '[{"siteGrading":"s57201"}]' in 'where clause' (SQL: select itemCode from item_statuses where itemCode = AM0201 and [{"siteGrading":"s57201"}] = M)'

via Chebli Mohamed