Low-Dose CaO₂ Enhanced Arsenite Coagulation via Elevating Solution pH and Persistently Oxidizing As(III) into As(V)

Abstract

As a low-cost and commonly used technology, coagulation usually encounters two difficulties in disposing arsenite (As(III)): (1) the removal efficiency is much lower than that of arsenate (As(V)), and (2) acidic conditions are quite detrimental to As(III) removal. Calcium peroxide (CaO 2 ) possesses dual functions of oxidizing pollutant and elevating solution pH. A CaO 2 -enhanced coagulation method was thus constructed in this study, aiming to develop a versatile coagulation strategy to treat As(III) pollution. Results showed that low-dose CaO 2 (20 mg/L) significantly improved As(III) coagulation, and cocoagulation with Ti(SO 4 ) 2 (10 mg Ti/L) achieved above 90% removal (initial As(III) was 1.0 mg/L) at pH 4.0−7.0. The oxidation function of CaO 2 played the dominant role, whereas the pH-elevation mechanism worked a more prominent role with the increase of the CaO 2 dose. The CaO 2 with longlasting oxidizing ability persistently converted As(III) into As(V), thus prompting the quick sequestration of resultant As(V) by coagulation reaction. The dried flocs could be further reused as As(III) adsorbent and reached a maximum adsorption capacity of 345.3 mg/g Ti (pH 5.0). CaO 2 -enhanced coagulation was a universal As(III) pollution control method with high efficiency and operational simplicity. For practical wastewater treatment, the dose of coagulant and CaO 2 should be further optimized according to As(III) concentration, coagulant type, and water quality parameters.

Acetylacetone promoted high-efficiency coagulation toward arsenite through a synchronous photooxidation process

“…It is well known that arsenic removal by coagulation can be completed within 1–2 minutes. 17 Therefore, it can be concluded that the As( iii ) in UV/AA + TS system was firstly oxidized to As( v ), and then the resultant As( v ) was quickly immobilized by the coagulated flocs.…”

“…In an oxidation–coagulation system, the oxidation of As( iii ) and coagulation removal of arsenic simultaneously occur. 17 Thus, the relationship between these two effects was investigated by determining the kinetic variations in the As( iii ) and As( v ) concentrations in solution and on the flocs. As shown in Fig.…”

“…For an initial As(III) concentration of 3.0 mg L −1 , the removal efficiency was only 46.2%. According to previous research, two reasons are mainly responsible for this low efficiency, as follows: (1) the conversion efficiency of As(III) to As(V) is low for high initial concentrations 17 and (2) the dose of coagulant was inadequate to treat high-load As(III). 33 3.2.2 Effects of irradiation time and coagulant/AA dose on As(III) removal.…”

“…It is well known that arsenic removal by coagulation can be completed within 1-2 minutes. 17 Therefore, it can be concluded that the As(III) in UV/AA + TS system was firstly oxidized to As(V), and then the 3.3.2 Immobilization mechanism of arsenic. AA can coordinate with metal ions through its hydroxyl structure.…”

“…[13][14][15][16] In addition, in our previous study, we found that when oxidation synchronously occurred in the coagulation system, a low dose of CaO 2 was sufficient to drastically improve the removal of As(III). 17 In this simultaneous oxidation-coagulation system, As(III) was persistently oxidized into AsIJV), and then the resultant As(V) was quickly immobilized on the coagulated floc. Compared with preoxidation-enhanced coagulation, the synchronous oxidation method is a more universal strategy to improve the coagulation efficiency because it can reduce the dose of oxidant and save the oxidation unit in practical application.…”