In　this　study,　a　novel　and　low-cost　seawater-modified　biochar　(SBC)　was　fabricated　via　the　pyrolysis　of　fir　wood　waste　followed　by　co-precipitation　modification　using　seawater　as　the　Ca/Mg　source.　The　co-precipitation　pH　was　a　vital　factor　during　modification,　and　the　optimal　pH　was　10.50　according　to　calculations　using　PHREEQC　2.5　and　experiments.　The　characterizations　indicated　that　Ca　and　Mg　were　loaded　on　the　SBC　as　irregular　CaCO3　and　nanoflake-like　Mg(OH)2,　respectively,　with　the　latter　dominating.　The　SBC　exhibited　a　high　maximum　adsorption　capacity　of　181.07　mg/g　for　phosphate,　calculated　using　the　Langmuir　model,　excellent　adsorption　performance　under　acidic　and　neutral　conditions　(pH　=　3.00–7.00),　and　remarkable　selectivity　against　Cl−,　NO3−,　and　SO42−.　The　presence　of　HCO3−　promoted　adsorption.　The　mechanisms　behind　phosphate　adsorption　involved　electrostatic　attraction,　ligand　exchange,　precipitation,　and　inner-sphere　complexation.　Mg,　rather　than　Ca,　was　served　as　the　main　adsorptive　sites　for　phosphate.　Additionally,　the　feasibility　of　treating　real-world　wastewater　was　tested　in　batch　(using　SBC　powders)　and　fixed-bed　column　(using　SBC　granules)　experiments.　The　results　indicate　that　the　SBC　powders　could　reduce　the　phosphate　concentration　from　1.26　mg　P/L　to　below　0.5　mg　P/L　at　a　low　dose　of　0.50　g/L,　and　the　SBC　granules　exhibited　a　high　removal　efficiency　with　excellent　recyclability;　the　capacity　still　remained　at　78.92%　of　the　initial　capacity　after　five　adsorption–desorption　runs.　Furthermore,　the　modification　process　almost　did　not　increase　the　production　cost　of　the　SBC,　which　was　estimated　to　be　0.41　$/kg.　Our　results　demonstrate　that　seawater　is　a　low-cost　and　efficient　modifier　for　biochar　modification,　and　the　resultant　SBC　demonstrates　great　potential　for　treating　actual　phosphate-containing　wastewater.　©　2022