The potential of powdered rock phosphate of some Iranian mines for removal of phosphate ions from aqueous solutions

Document Type : Research Paper

Authors

• Samaneh Jafary
• Ebrahim Adhami
• Hamidreza Owliaie
• Mohamad Sedghi asl

Soil Science department, College of Agriculture, Yasuj university, Yasuj, Iran.

Abstract

Keywords

• Phosphorus release
• Sorption isotherms
• Sorption Kinetics
• Freundlich equation
• Langmuir equation

Main Subjects

• ..

The potential of powdered rock phosphate of some Iranian mine for removal of phosphate ions from aqueous solutions

EXTENDED ABSTRACT

Introduction

Natural apatite, known as powdered phosphate rock (PPR), is a low price fertilizer material that has been attempted to be used instead of commercial P fertilizers. Though the direct reaction of P with PPR is not considered important, it is reported that natural apatite could sorb soluble P, either by precipitation or adsorption mechanisms. The present study was conducted to evaluate the phosphate sorption ability of finely ground phosphate rock of four Iranian mines and to quantify the q/I relationships and kinetics of P retention with PPR.

Materials and Methods

Four PPR samples of Iranian mines were tested, two samples from Esfordi mine of Yazd province, one from Chadormalu mine of Yazd province, and one from a mine, near Yasuj. In duplicate, the phosphorus sorption isotherm was evaluated by 1:20 ratio of PPR:P solution containing 10, 50, 100, 200, 400, 600, 800 and 1000 mg P L^-1 (as KH₂PO₄). Sorption data was fitted with the linear form of Langmuir, and Freundlich models. Following the sorption the release of sorbed P was evaluated by equilibrating with 20 ml CaCl₂ 0.01 M. The Fourier transform infrared (FTIR) spectra of the untreated PPR samples and those treated with 1000 mg P L^-1 were recorded using a JASCO FTIR 460 plus spectrometer. The kinetics of P sorption on PPR samples was studied with two initial P concentrations (50 and 100 mg P L^-1) in 0.01 M CaCl₂ as the background solution with a ratio of 1: 20 of PPR:P solution under laboratory temperature (20-25 ºC) and in equilibrium time of 1, 2, 4, 8, 16, 24, and 48 h shaking. Kinetic equations, including zero order, first order, Elovich, parabolic diffusion, and power function models, were used to fit the kinetic sorption.

Results

The Ca and P content of the PPRs ranged from 41.3 to 62.6% CaO (29-44% Ca); and 6.90 to 24.50% P₂O₅ (3-11% P), respectively. Iron content was the highest in the raw PPR of Esfordi (around 20% Fe₂O₃), and the other PPRs contained 3-5 % Fe₂O₃. P sorption ranged from 144 to 12080 mg P kg^-1 PPRs, and the highest P sorption occurred in Chadormalu, Yazd, the sample that reached up to 12000 mg kg^-1. The release of sorbed P was 18-263 mg kg^-1, and the highest release belonged to the PPR sample of Chadormalu, Yazd. The release quantity of the sorbed P increased linearly up to the initial concentration of 600 mg P L^-1 and tended to remain constant at higher concentrations. Maximum % P release (around 15 % of the sorbed P) occurred at 10 mg P L^-1. It drastically decreased by increasing P concentration and reached a minimum of about 2% above 100 mg P L^-1. Phosphorus sorption data was well fitted to Freundlich and Langmuir isotherm models. The intercept of the Freundlich equation, k_F, was 129-157 mg kg^-1 P. The least K_F belonged to the Esfordi Yazd PPR, and the highest one was observed in Chadormalu Yazd's PPR. The maximum retention capacity (b_L) calculated from the Langmuir equation ranged from 15384 to 22222 mg kg^-1 PPR. FTIR analysis showed that The strongest peak occurred on 1045 cm^-1 for all samples, attributed to the asymmetric n₃ mode of phosphate groups. The peaks in »466 n₂ (bending) mode of phosphate groups, 570 and 609 asymmetric n₃ mode, and P-O mode were evident in all samples. The quantity of P sorption vs. time (1 to 48h) ranged 540-1000 and 1123-2000 mg kg^-1 on 50 and 100 mg P L^-1, respectively. Phosphorus sorption by different PPRs, was initially fast and decreased by the increase of contact time. For all PPRs, the maximum sorption rate occurred at the first initial contact time (1 h) and decreased gradually. More than half of the final sorption quantity occurred at the initial time (1h). The content of P sorption was in the following order: Yasuj (540 mg kg^-1) < Chadormalu (589 mg kg^-1) -1)< raw Esfordi (742 mg kg^-1); which was consistent over time. The raw Esfordi PPR had the highest P sorption rate and caused the removal of almost total soluble P in just 8 h. A similar trend with a slower kinetic rate was observed for the purified Esfordi PPR sample, which eliminated total soluble P in 16 h.

Conclusion

Results of the present study suggest that finely ground PPRs could rapidly retain a large quantity of soluble P, which is rarely reversible. Results presented here may be useful in the fixation of soluble P in sewage sludge or sewage effluent to prevent P transport in the environment and consequent water resources eutrophication.