“…54 At low ionic strengths and at circumneutral to basic conditions, Sr( ii ) can sorb to mineral surfaces including Fe-(oxy)hydroxides and clays due to their large cation exchange capacity and surface area. 55–60 Here, Sr( ii ) typically forms outer-sphere complexes in competition with other cations (such as H + , Na + , K + , Mg 2+ ), 61–65 and Sr( ii ) bound via outer sphere sorption is considered susceptible to desorption and remobilisation with changes in pH and ionic strength. 65–67 Simple electron donors (such as acetate, lactate or glycerol) which promote bioreduction processes typically do not impact Sr( ii ) removal from solution 64,68,69 unless other biogeochemical processes impact on pH and lead to alkaline conditions.…”