Control of phosphorus release from sediment by hydrous zirconium oxide combined with calcite, bentonite and zeolite.

This study investigated the effectiveness and mechanism for the control of internal phosphorus (P) liberation from sediment by hydrous zirconium oxide (HZrO) combined with calcite, bentonite and zeolite. The results suggested that coexisting calcite, calcium-modified bentonite (CaBT) and calcium-modified zeolite (CaZ) all had the ability to promote the adsorption of phosphate (PO) onto HZrO. The mechanisms of PO elimination by HZrO/calcite mixture involved the adsorption of PO on calcite, the precipitation of PO with Ca, and the inner-sphere complexation of PO with HZrO. The amendment of sediment with HZrO/calcite, HZrO/CaBT or HZrO/CaZ mixture can effectively prevent the sedimentary P release, and the immobilization of mobile P in the sediment and the uptake of dissolved reactive P (DRP) from the interstitial water by the amendment material played a key role in the control of P release from sediment by the combined amendment. Capping sediment with HZrO/calcite, HZrO/CaBT or HZrO/CaZ mixture also can effectively intercept sediment P release, and the formation of P static layer attributed to the uptake of interstitial water DRP and DGT (diffusive gradient in thin-films)-unstable P in the upper sediment by the capping material was a key to the inhibition of sedimentary P migration into the overlying water by the combined capping. The great majority of P immobilized by the HZrO/calcite, HZrO/CaBT or HZrO/CaZ combined covering layer is stable P and it has a low re-releasing risk under dissolved oxygen-deficit and pH 5-9 condition. The stability of P bound by the combined covering layer was larger than that by the single HZrO covering layer. The results of this research show that the combined use of HZrO and calcite, HZrO and CaBT, or HZrO and CaZ as a capping material has great potential in the reduction of sediment P loading.