Gas-bubbled nano zero-valent iron process for high concentration arsenate removal.

In this study, batch experiments were performed to investigate a novel process for high concentration arsenate removal in the presence of air and/or CO(2) bubbling. The pretreatment step, CO(2) bubbling at 300 mL/min for 5 min, was taken to adjust the solution pH to an acidic environment, followed by air bubbling at 300 mL/min for 10 min to increase dissolved oxygen in the solution. In the treatment period, the nano-scale zero-valent iron was applied to remove aqueous arsenate of 3000 µg/L, while the treatment system was continuously bubbled by 300 mL/min of air. Such a process resulted in outstanding performance in arsenate removal. Furthermore, in the field groundwater application, the arsenate removal rate for the proposed process was 5 times faster than the rate measured when the system was pretreated by acidic chemical species only.

Impact of selected solution factors on arsenate and arsenite removal by nanoiron particles.

INTRODUCTION: The nano-scale zero-valent iron (NZVI) was used for the removal of arsenite (As(III)) and arsenate (As(V)) in aqueous solution. Batch experiments were conducted to investigate the effects of initial pH, initial arsenic concentration, dissolved oxygen (DO), and ratio of As(III)/As(V) on arsenic removal. MATERIALS AND METHODS: The NZVI synthesized by using NaBH4 and FeCl3 was put into use right after its synthesis. The arsenic treatment system of recirculation mode consists of a reactor with a liquid volume of 4.4 L, which provides dual function of reactants reaction and particles settling in one unit. RESULTS AND DISCUSSION: Consequently, the pseudo-first order rate equations can be used to describe the removal kinetics for As(V) at pH 4 and 7, while the pseudo-second order reaction was observed for As(V) at pH 9 and As(III) at all pH's studied. Arsenic removal rates of both As(V) and As(III) were lower in the system with lower DO. The rate of As(III) removal decreased with the increase of its initial concentration. In contrast, the removal rate of As(V) still remained significantly high as its initial concentration increased. CONCLUSIONS: This study reveals that low pH and high DO will favor arsenic removal. With the mixture of As(III) and As(V), the total arsenic was removed faster than solution containing As(III) or As(V) alone. In addition, the mixture with higher fraction of As(V) resulted in higher arsenic removal.

Evaluation of the multiple-ion competition in the adsorption of As(V) onto reclaimed iron-oxide coated sands by fractional factorial design.

This study describes the competitive effects of selected ions and natural organic matter on As(V) removal using reclaimed iron-oxide coated sands (RIOCS) in the single- and multi-ion systems. A 2(7-3) factional factorial experimental design (FFD) was employed for screening main competitive factors in this adsorption process. As a result, the inhibitive competition effects of the anions on As(V) removal in the single ion system were in the following sequence: PO(4)(3-)>SiO(3)(2-)>HCO(3)(-)>humic acid (HA)>SO(4)(2-)>Cl(-), whereas the cation Ca(2+) was observed to enhance the As(V) removal. In addition, the optimum initial pH for As(V) removal in single-ion system was 5. Based on the estimates of major effects and interactions from the FFD, PO(4)(3-), SiO(3)(2-), Ca(2+) and HA were important factors on As(V) removal in the multi-ion system. The promoters for the As(V) removal were found to be Ca(2+) and, to a lesser extent, SO(4)(2-). The competitive effects of these ions on As(V) removal were in the order of PO(4)(3-), SiO(3)(2-), HA, HCO(3)(-), and Cl(-). In the single ion system, the efficiencies of As(V) removal range from 75% to 96%, much higher than those in the multi-ion system (44%) at the initial pH 5. Clearly, there were some complex anion interactions in the multi-ion system. To promote the removal of As(V) by RIOCS, it is proposed to lower the pH in the single-ion system, while in the multi-ion system, the increase of the Ca(2+) concentration, or decreases of PO(4)(3-), SiO(3)(2-) and HA concentrations is suggested.

Removal of As(V) and As(III) by reclaimed iron-oxide coated sands.

This paper aims at the feasibility of arsenate and arsenite removal by reclaimed iron-oxide coated sands (IOCS). Batch experiments were performed to examine the adsorption isotherm and removal performance of arsenic systems by using the IOCS. The results show that the pH(zpc) of IOCS was about 7.0 +/- 0.4, favoring the adsorption of As(V) of anion form onto the IOCS surface. As the adsorbent dosage and initial arsenic concentration were fixed, both the As(V) and As(III) removals decrease with increasing initial solution pH. Under the same initial solution pH and adsorbent dosage, the removal efficiencies of total arsenic (As(V) and As(III)) were in the order as follows: As(V)>As(V)+As(III)>As(III). Moreover, adsorption isotherms of As(V) and As(III) fit the Langmuir model satisfactorily for the four different initial pH conditions as well as for the studied range of initial arsenic concentrations. It is concluded that the reclaimed IOCS can be considered as a feasible and economical adsorbent for arsenic removal.