Arsenic　existed　concomitantly　with　minerals　containing　precious　or　non-ferrous　metals,　in　which　arsenic-containing　wastewater　(ACW)　is　inevitably　generated　during　the　process　of　smelting.　In　the　treatment　of　ACW,　the　precipitation　method　using　sulphide　is　widely　used,　but　the　resulting　arsenic　sulphide　slag　poses　a　risk　of　secondary　pollution.　In　this　work,　a　reduction　strategy　was　developed　to　transform　arsenic　sulphide　slag　into　elementary　arsenic　under　hydrothermal　conditions　by　using　sodium　formate　as　the　reductant,　with　the　aim　to　treat　the　slag　and　recycle　As.　The　experimental　results　show　that　As(0)　can　be　obtained　by　one-step　hydrothermal　reduction　by　sodium　formate　at　temperature　of　200　℃　and　pH　4,　and　the　purity　of　elementary　arsenic　produced　under　the　optimum　conditions　is　higher　than　90%.　Mechanism　analyses　reveal　that　the　reduction　process　of　As2S3　to　As(0)　experiences　two　steps,　where　amorphous　As2S3　primary　particles　first　transform　and　crystallize　into　partially　reductive　AsS　in　a　bulk　size,　and　then　AsS　is　further　reduced　into　As(0)　through　a　collapse　process　to　form　small　particles.　This　work　provides　a　recycling　strategy　for　the　efficient　conversion　of　arsenic　sulphide　slag　to　As(0)　for　potential　applications.　©　2023　The　Institution　of　Chemical　Engineers