The man, in his 50s, is believed to be from the Maidstone area of Kent.
The clockwise carriageway leading to the Dartford crossing will remain shut until further notice following the crash between junction 30 and junction 31, police said.
The arrested female driver, aged in her 30s and from Romania, remains in custody and is being interviewed.
Paramedics attended and both carriageways were shut to allow an air ambulance to land. Police said they were now carrying out an investigation.
The slip road from Grays on to the clockwise carriageway is also closed.
Motorists are advised to seek an alternative route, although the anti-clockwise carriageway has now reopened.
Traffic congestion is very heavy but vehicles initially caught at the scene of the collision have been released, police said.

The Main Stand would be expanded to three tiers, while work on the Anfield Road Stand is also proposed.
In October 2012, the club said they would redevelop the ground rather than build a new one on nearby Stanley Park.
Liverpool managing director Ian Ayre said: "Good progress has been made so far and we are proud to be able to unveil our plans."
Part of the proposed redevelopment would result in the memorial to the 96 Liverpool fans who died in the Hillsborough disaster of 1989 moved into the rebuilt Main Stand.
If the proposals to expand both stands go ahead, the ground's capacity would increase to 58,800.
Local residents raised concerns about any expansion in 2012 because it would involve the demolition of some houses near the ground.
The club said that discussions with local property owners were "progressing well" and all parties are close to agreement but added details were still to be finalised.
Following a consultation process, the club expects to present its finalised plans and submit a planning application to the city council this summer.
If successful, they expect construction to begin early next year with work completed by the start of the 2016-17 season.
See the future plans for Anfield.

They include a parade of Afro-American servicemen in Stowmarket, Suffolk.
Eighth in the East has been given Â£500,000 by the Heritage Lottery Fund to document the history of 70 sites.
David Cain, project manager, said: "The images are important as they show what we have lost and what replaced these people and buildings in our landscape."
Some of the airfields have become airports such as Norwich, some have new commercial uses, while others have become fields again.
The public is being asked to contribute any historic photographs they have and to go out and take photos of the sites as they are now.
Eighth in the East said the images will contribute to its ongoing educational programme and a touring exhibition due to get begin later this year and into 2016.
The Now and Then project organisers said they were particularly keen to receive old and current photos of airfields in Essex, Cambridgeshire, Hertfordshire and Bedfordshire.
A lorry driver from Romania has been arrested after a motorcyclist died in a collision on the M25 in Essex.
[NEXT_CONCEPT]
Liverpool have revealed plans to increase Anfield's capacity from 45,500 to almost 59,000.
[NEXT_CONCEPT]
Previously unseen photographs of East Anglian airfields when they were used by US forces in World War Two have been unearthed.