Belgian　coastal　sediment　serves　as　an　important　sink　for　trace　elements,　yet　a　systematic　study　covering　a　wide　range　of　elements　including　redox-sensitive　metals　(Fe,　Mn,　and　Co),　cationic　trace　metals　(Cd,　Pb,　Ni,　Cu,　and　Zn),　oxyanions　(P,　V,　As,　and　Mo),　and　sulfide　has　not　been　performed　and　the　mechanisms　controlling　their　mobilization　were　not　investigated.　Here,　a　passive　sampling　technique,　Diffusive　Gradients　in　Thin-films　(DGT),　was　used　in　situ　to　obtain　high　resolution　concentration　profiles　of　these　elements　in　the　sediment　porewater.　Our　results　revealed　two　mobilization　mechanisms　of　cationic　trace　metals　and　oxyanions　in　Belgian　coastal　sediments,　both　strongly　linked　to　the　cycling　of　Fe.　Mobilization　of　Co,　Pb,　Ni,　and　Cu　is　controlled　by　electrogenic　sulfur　oxidation,　acidification　of　the　porewater　and　dissolution　of　FeS,　while　that　of　oxyanions　(P,　V,　and　As)　is　controlled　by　reductive　dissolution　of　Fe　oxyhydroxides.　Constant　cationic　trace　metal　to　Fe　molar　ratios　were　established　in　FeS,　while　the　oxyanion　to　Fe　ratios　in　Fe　oxyhydroxides　differ　significantly　between　sampling　stations,　which　is　primarily　caused　by　competing　effects.　We　found　no　evidence　that　cationic　trace　metal　mobilization　was　related　to　Fe　oxyhydroxides,　or　oxyanion　mobilization　to　FeS.　This　suggests　that　particulate　organic　matter　forms　the　major　pathway　for　cationic　trace　metal　input　in　coastal　sediments　and　that　oxyanions　will　not　be　incorporated　in　FeS　but　form　their　own　oxyanion-sulfide　compound.　These　findings　will　contribute　to　a　better　understanding　of　the　mobilization　mechanisms　of　cationic　trace　metals　and　oxyanions　in　coastal　sediments,　and　of　their　biogeochemical　cycling　in　coastal　ecosystems.　©　2020　The　Author(s)