Sinteza, karakterizacija i primena sorbenata na bazi gvožđa i mangana za uklanjanje arsena iz vode / Synthesis, characterisation and application of iron and manganese based sorbents for arsenic removal from water

The presence of inorganic arsenic in groundwater used for drinking water supply is a global problem. Different techniques such as oxidation, coagulation, adsorption, ion exchange, and membrane filtration have been developed and applied for arsenic removal from aqueous media. Among these technologies, adsorption is regarded as one of the most promising approaches to remove arsenic from water because of its high efficiency, low cost, simplicity of operation. Although many sorbents for arsenic removal are available on the market, there is still a need to identify and develop new lowcost sorbents which are highly effective in removing both oxidation states of arsenic, As(III) and As(V). This dissertation therefore presents the synthesis and characterization of ten new iron and manganese based sorbents specifically developed for effective As removal. The Fe-Mn binary oxides were prepared with Fe:Mn molar ratios of 1:1, 3:1, 6:1 and 9:1, while four heterogeneous magnetic composites (Mag, Mag-Fe, Mag-Mn, Mag-FeMn) were synthesized by combining the heterogeneous nucleation technique with precipitation. The remaining two materials, Chit-FeMn and GAC-FeMn, were created by modifying the Chitosan and GAC biopolymers with Fe-Mn binary oxide (Chit-FeMn and GAC-FeMn). Multiple techniques were applied to determine the physical and chemical characteristics of the resulting sorbents (including SEM/EDS, XRD, FTIR and BET analyses). In order to establish which sorbents show the greatest promise for application during drinking water treatment, the sorption capacity of the sorbents, the Jasmina Nikić Doktorska disertacija 229 As(III) and As(V) sorption mechanisms, and the impact of various factors relevant to arsenic sorption, including the regeneration potential and the reuse potential of the sorbents, were all investigated in batch experiments. During the physical characterisation, the Fe-Mn binary oxides and magnetic materials were found to have relatively large specific surface areas (109-300 m2 /g) and mesopore volumes (0.144-0.403 cm3 /g). A large specific surface area and microporous structure was observed for GAC-FeMn (996 m 2 /g; 0.394 cm3 /g). In comparison with the other synthesized sorbents, Chit-FeMn has the smallest specific surface area and pore volume (1.99 m2 /g; 0.014 cm3 /g). XRD analyses of the Fe-Mn binary oxides indicated that the phase structure of the synthesized Fe-Mn binary oxides was similar to ferrihydrate, while the phase structure of the magnetic materials showed a good agreement with the XRD diffractogram of maghemite. Investigations into As sorption process kinetics have shown that the sorption mechanism for both As(III) and As(V) on the synthesized sorbents is a combination of surface hemisorption, which takes place through the boundary layer of sorbent particles, and intracellular diffusion. The FTIR analyses confirmed that arsenic sorption was accomplished through the interactions of the hydroxyl groups of iron present on the surface of the sorbents and arsenic. In contrast to the sorption of As(V), it was also shown that As(III) sorption onto sorbents containing manganese oxides (Fe-Mn binary oxides, Mag-FeMn, Mag-Mn, Chit-FeMn and GAC -FeMn) takes place in two steps. In the first step As(III) is oxidized to As(V), while in the second step, the oxidized As(V) forms complexes on the surface of the sorbents via ligand exchange. The Fe-Mn binary oxide sorption capacities (expressed as Kd values) for As(III) followed the trend Fe-Mn 3:1 > Fe-Mn 1:1 > Fe-Mn 6:1 > Fe-Mn 9:1, whereas the As(V) trend was Fe-Mn 6:1 > Fe-Mn 3:1 > Fe-Mn 9:1 > Fe-Mn 1:1. In the magnetic composites, the largest Kd value for As(III) and As(V) was obtained for Mag-FeMn. Generally, the Kd values for As(III) in the magnetic composites decreased in the series: Mag-FeMn > Mag-Mn > Mag > Mag-Fe. Similarly, the affinity of the sorbents for As(V) was as follows: Mag-FeMn > Mag > Mag-Fe > Mag-Mn. In comparison to the nonimpregnated materials, the Kd values were much higher for the coated materials, Chit-FeMn and GAC-FeMn, demonstrating the significant advantage Fe-Mn binary oxides provide in increasing As(III) and As(V) adsorption capacities. One of the most problematic limiting factors in applying adsorption technologies during drinking water treatment is the presence of other water constituents which interfere with the adsorption process. Investigations into the inhibitory effect of competive anions on the adsorption of both forms of arsenic revealed that phosphates were the worst offenders in terms of Jasmina Nikić Doktorska disertacija 230 reducing the arsenic removal efficacy of sorbents investigated. From the largest to the smallest negative influence of the anions investigated, for As(III) and As(V) adsorption on Fe-Mn binary oxides, magnetic materials and Chit-FeMn, as well as As(V) adsorption on GAC-FeMn, the order was: phosphates > silicates > carbonates > sulfates > nitrates > chlorides, with the presence of the latter two anions proving almost irrelevant to the As adsorption process. Similarly, the negative influence of anions on As(III) sorption on GAC-FeMn was: phosphates > silicates > sulphates > carbonates > nitrates > chlorides. Another issue with applying adsorption in real treatment conditions is the need to regularly regenerate and/or replace the spent sorbent. In this work, a simple and efficient process for sorbent regeneration is demonstrated. This regeneration process can be applied to the sorbents investigated multiple times, and uses an NaCl-NaOH-NaOCl mixture, or 0.1 M and 0.5 M NaOH solutions. This finding is of great importance from an ecological and economic point of view. The minimum reduction in the sorption capacity for both As(III) and As(V), after five sorption-regeneration cycles, was found in the binary oxide with a 3:1 Fe:Mn molar ratio and Mag-FeMn. Arsenic sorption behaviour was also investigated using real groundwater samples, with the results demonstrating the great potential of 3:1 Fe-Mn binary oxide and Mag-FeMn. However, Chit-FeMn and GAC-FeMn were less effective at adsorbing As from the groundwater samples. Based on the results presented in this dissertation, it can be concluded that the synthesized sorbents, especially the Fe-Mn binary oxides and magnetic composites, and Mag-FeMn in particular, can be efficient and economical alternatives to expensive commercial sorbents and other sophisticated As removal technologies. The high oxidation and sorption capacity of these materials, which ensure the simultaneous removal of arsenic with both oxidation states, is a large advantage for these sorbents and makes them very attractive and promising for application in drinking water treatment. An additional benefit of the magnetic sorbents, primarily Mag-FeMn, is the ease with which they may be separated from the aqueous medium, allowing for simple recirculation within a system. Similarly, the main advantages of the synthesized Chit-FeMn and GAC-FeMn are reflected in their application as effective filtration media.