Gastropod shell column reactor as on-site system for phosphate capture and recovery from aqua system

Abstract

Gastropod shell (GS), whose distribution is worldwide and mineralogical assemblage is aragonite, was packed in a glass column and evaluated for phosphate capture and recovery from aqua system. The column reactor was operated in a batch-continuous mode, considered as a close approximation to a practical water treatment system, for 30 days. The time–concentration profile parameters were derived at different influent phosphate concentrations and hydraulic residence time (HRT) to determine the performance efficiency of the reactor column. The mechanism of phosphate removal was investigated and phosphate fractionation protocol was employed to elucidate the form and pattern of phosphate distribution in the spent reactor. Variations in influent phosphate concentration and HRT had minimal influence on the time–concentration profile parameters and the performance efficiency of the column reactor within the process variables studied. The first order rate constant (k1) showed that increase in the HRT of the influent in the SS column reactor does not enhance the phosphate removal. The value of the saturation index (SI) obtained over the 30 days period were all positive and the thermodynamic parameter (ΔG) was <0 which is an indication of supersaturation, the significant role of precipitation as a mechanism of phosphate removal in the reactor and spontaneity of the precipitation reaction. The order of distribution of each fraction of P, relative to the total phosphorus (T-P) in the phosphate laden SS, is as follows: dicalcium phosphate (Ca2-P) (42.5%)>ten-calcium phosphate (Ca10-P) (40.50%)>octacalcium phosphate (Ca8-P) (16.65%)>occluded phosphate (O-P) (0.35%). The simple appraisal of the fertilizer value of the spent reactor showed that it would preferentially serve as a soil conditioner cum fertilizer in acidic soil.