Effect Of Natural Organic Matter On Arsenic Removal From Groundwater By Modified Granular Natural Siderite

Arsenic contamination in groundwater has become an issue of worldwide concerns because of its strong toxicity and carcinogenicity to human health. In China, high As groundwater with As concentration greater than Chinese drinking water standard had been found in more than 40 counties, which usually lead to chronic arsenic poisoning to local residents. Therfore, high arsenic groundwater has been attached attention of public and government. Natural organic matter(NOM) is a ubiquitous substance in groundwater.It has been well-known that NOM possesses numerous functional groups which enable to form aqueous complexes with metals and metalloids. Therefore, the presence of NOM in groundwater plays an important role in their fate and transformation.Modified granular natural siderite(MGNS) was selected as adsorbent to investigate effects of NOM on arsenic(As) removal. Through a serious of batch tests, effects of NOM type and concentration on arsenic(As) removal were evaluated in(NOM-MGNS)- As system and(NOM-As)- MGNS system. Experimental data were fitted to Lagergren dynamics and adsorption isotherm model. Besides, adsorption mechanisms were assessed by using Fourier transformed infrared(FTIR) spectroscopy, high-performance size exclusion chromatography- Inductively Coupled Plasma Mass Spectrometry(HPSEC-ICP-MS) and extended X-ray absorption fine structure spectroscopy techniques(EXAFS).Results showed that the presence of HA with low concentrations(0 ~ 20 mg/L DOC) exhibited little effect on As(III) adsorption, while a slight inhibition to As(V) adsorption was observed. It was found that kinetic data were better fitted to Lagergren pseudo-second-order model and isotherm data to Langmuir isotherm. With increasing NOM concentrations(>40 mg/L DOC), little effect on As(III) adsorption was still observed. However, removal of As(V) was inhibited more evidently. There are no redox reaction during As(III)/As(V) adsorption. Compared with humic acid(HA), the inhibiting effect of fulvic acid(FA) on As adsorption is much stronger. In comparison with As(III), the inhibiting effect of NOM was more evident on As(V).Mechanisms of effect of natural organic matter on As adsorption on MGNS were evaluated, showing that NOM can form complexes with Fe oxides/hydroxides, e.g., Fe(II)-NOM and Fe(III)-NOM macromolecules. As a result, the complexes was occupying the surface sites on the MGNS. On the other hand, NOM is negatively charged and NOM–coated MGNS would alter the surface charge to form electrostatic repulsion with arsenic, which can also constrain the arsenic removal efficiency. Compared with the only adsorption reaction on MGNS and As(V), a series of reactions such as oxidation and reduction reaction、adsorption reaction was observed on the surface of MGNS and As(III), so that the inhibiting effect of NOM on As(V) adsorption is much stronger.