Compacted　clay　is　employed　as　the　buffer　material　for　landfills,　and　multiple　ions　are　dissolved　in　the　leachate　restricted　by　the　compacted　clay　layer.　The　membrane　efficiency　is　an　important　indicator　to　assess　the　barrier　properties　of　the　compacted　clay　layer　and　is　measured　through　membrane　tests.　However,　most　membrane　tests　are　currently　conducted　with　a　single　solute　solution,　which　does　not　reflect　the　mixed　solution　characteristics　of　leachates.　To　assess　the　membrane　efficiency　of　compacted　clay　under　mixed　solution　conditions,　13　membrane　tests　were　conducted　on　a　bentonite-amended　soil　using　KCl–NaCl　mixed　solutions,　KCl–CaCl2　mixed　solutions,　and　KCl–AlCl3　mixed　solutions　with　different　mixing　ratios　at　a　total　concentration　of　20　mM.　Nuclear　magnetic　resonance　(NMR)　tests　were　conducted　on　the　soil　specimen　after　the　membrane　tests　to　investigate　the　micromechanism　of　the　membrane　behavior　under　mixed　solution　conditions.　Results　indicate　that　the　membrane　efficiency　increased　with　the　mixing　ratio　of　Na+　but　decreased　with　the　mixing　ratio　of　Ca2+　or　Al3+.　In　the　13　membrane　tests,　the　lowest　membrane　efficiency　was　achieved　when　the　specimen　was　tested　with　pure　AlCl3　solution.　The　relationship　between　the　membrane　efficiency　and　mixing　ratio　was　also　investigated　at　the　microscopic　scale.　As　the　ion　valence　increases,　the　diffuse　double　layer　thickness　is　smaller　and　the　proportion　of　macropores　is　larger,　as　verified　by　NMR　tests.　©　2024　Elsevier　B.V.