Sulphur and sulphur compounds showed to be very toxic, so have been in the west and Tunisia also already banished from medical use ( sulfamide...) and agriculture ( sulphur powder as pesticid ) !
But as you see : the port officers do not have enough general culture to know it and take care ! They use for sulphur powder same loading means as rocks !
Especially in windy weather , sulphur flows to the sea , and I dived nearby and saw the sea bottom : all covered by a thick layer of yellow ( it lasted years and decades and still running ). If marine life is not intoxicated by the chemical through inhalation , sulphur will surely become a mechanical hazard preventing little creatures from living and feeding on the sea bottom .
I think this makes of Tunisia a serious focus of ecology : these amounts of sulphur will surely spread in the Mediterranean sea that is a common harbor for three continents Africa ( Algeria , Morocco , Egypt ...) , Europe ( France , Italie, Spain...) and Asia ( Lebanon, Syria... ).
It is serious because there is at least another ( may be likewise ) known loading area in Gabes , further southern town at 70 km From Sfax where the image was taken.