Abstract
Ball milling and ultra-sonication size reduction procedures were applied to granular ferric hydroxide (GFH) to obtain two micro-sized adsorbents. These two adsorbents and GFH were investigated to improve the removal of phosphates from water. The size reduction procedures, using the milling method, allowed a reduction of size from 0.5–2 mm to 0.1–2 µm and total disaggregation of the GFH structure. Using an ultra-sonication method yielded a final size of 1.9–50.3 µm with partial disaggregation. The Langmuir model correlated well with the isotherms obtained in batch equilibrium tests for the three adsorbents. The maximum adsorption capacity (qmax) for the milled adsorbent was lower than GFH, but using ultra-sonication was not different from GFH. The equilibrium adsorption of two wastewater samples with phosphate and other anions onto the GFH corresponded well with the expected removal, showing that potential interferences in the isotherms were not important. Batch kinetics tests indicated that the pseudo second-order model fitted the data. Long-term adsorption capacity in kinetics (qe) showed the same trend described for qmax. The application of milling and ultra-sonication methods showed 3.5- and 5.6-fold increases of the kinetic constant (k2) versus the GFH value, respectively. These results showed that ultra-sonication is a very good procedure to increase the adsorption rate of phosphate, maintaining qe and increasing k2.
Highlights
Several Circular Economy approaches applied to the adsorption of phosphate have been considered in the literature
The present study investigated the potential of size reduction of a low-cost granular adsorbent using milling and ultra-sonication on the batch-scale adsorption of phosphate
While the milling procedure was more effective for size reduction, the total elimination of the initial granular ferric hydroxide (GFH) structure led to a notable decrease in the specific surface area and qmax and qe
Summary
Several Circular Economy approaches applied to the adsorption of phosphate have been considered in the literature These alternatives include the recovery of phosphorus and water from several wastewater streams [1,2,3,4], the use of low-cost products or recovered waste as sorbents [5,6,7,8,9,10,11], and the study of the re-use of phosphate-loaded sorbents [2,3,11,12,13]. The adsorption of phosphates from aqueous solutions and wastewaters onto the GFH is usually performed in fixed-bed type contactors [18] This system is operated, but multiple references show early breakthrough curves of phosphate in fixed-bed column assays [14,19,20]. This behavior is linked with sorbent capacities lower than in batch tests and is attributed to kinetic mass-transfer limitations [11,14,21]
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