Evaluation of dried amorphous ferric hydroxide CFH-12® as agent for binding bioavailable phosphorus in lake sediments

Metal hydroxides formed from aluminum (Al) and iron (Fe) salts can be used as phosphorus (P) adsorbents in lake restoration, but the application entails problems in low-alkaline lakes due to acid producing hydrolysis and potential formation of toxic metal ions. Therefore, we tested the potential of applying CFH-12® (Kemira) - a dried, amorphous Fe-oxide with no pH effect - in lake restoration. Since Fe³⁺ may become reduced in lake sediments and release both Fe²⁺ and any associated P we also evaluated the redox sensitivity of CFH-12® in comparison with freshly formed Fe(OH)₃. CFH-12® was added to undisturbed sediment cores from three Danish lakes relative to the size of their mobile P pool (molar Fe:P_Mobile dose ratio of ~10:1), and P and Fe fluxes across the sediment-water interface were compared with those from untreated cores and cores treated with freshly formed Fe(OH)₃. Under anoxic conditions, we found that CFH-12® significantly reduced the P efflux from the sediments (by 43% in Lake Sønderby, 70% in Lake Hampen and 60% in Lake Hostrup) while the Fe²⁺ efflux remained unchanged relative to the untreated cores. Cores treated with freshly formed Fe(OH)₃ retained more P, but released significantly more Fe²⁺, indicating continued Fe³⁺ reduction. Finally, experiments with pure phases showed that CFH-12® adsorbed less P than freshly formed Fe(OH)₃ in the short term, but was capable of adsorbing up to 70% of P adsorbed by Fe(OH)₃ over 3months. With product costs only 30% higher than Al salts we find that CFH-12® has potential for use in restoration of low-alkaline lakes.