Arsenic removal from contaminated waters by Fe-based (hydr)oxides and its phytoavailability in soil-plant system
Creators: name: Azam, Shah Md. Golam Gousul
Publisher: name: Università degli Studi di Napoli Federico II
Year: 2015

description: Arsenic is considered as being synonymous with toxicity. Alarming As concentrations in natural waters is a worldwide problem. With greater public concern of As poisoning on human health, the developing regulatory guidelines and remediation technologies for mitigating As-contaminated ecosystems is an important issue. Urgent steps must be taken to reduce these impacts by providing access to safe water as a basic human right. Delaying mitigation is increasing death and disease. 
Layered double hydroxides (LDHs) show excellent capacity to sorb and exchange anions. Although arsenic (As) has a high affinity for sorbents containing Fe, only few studies have been carried out on the sorption of arsenite [As(III)] onto a Fe-based LDH. In this work we studied the sorption of As(III) onto a LDH containing Mg and Fe (Mg–Fe-LDH), as affected by: (i) pH; (ii) the presence of increasing concentrations of organic [citrate (CIT) and oxalate (OX)] and inorganic [phosphate (PO4), selenite (SeO3) and sulphate (SO4)] ligands, (iii) the effect of residence time on the desorption of As(III) by these ligands, and; (iv) the kinetics of desorption of As(III) by PO4.
The As(III) sorption isotherms, carried out at different pHs, indicated a good affinity of the harmful oxyanion for the sorption sites of the Mg–Fe-LDH. This material was able to remove efficiently As(III) from contaminated solutions over a wide range of pH, but more in acidic than in alkaline systems. Competing ligands differently prevented the As(III) sorption on Mg–Fe-LDH, according to the following order: SO4 &lt; OX &lt;&lt; SeO3 &lt; CIT &lt; PO4. The desorption of As(III) by these anions decreased with increasing As(III) residence time on LDH surfaces. A comparison between the sorption/desorption of As(III) and As(V) on/from Mg–Fe-LDH in the presence or absence of anions highlights that less As(III) than As(V) is sorbed, whereas more As(III) than As(V) is desorbed by all the selected organic and inorganic ligands, but PO4.
In recent decades, Fe-(hydr)oxides (e.g., Ferrihydrite and Goethite) have attracted substantial attention for their potential use as As sorbents. The aim of the work was to study the As(III) removal from contaminated waters by Fe-based minerals, by using Ferrihydrite [non-crystalline Fe-(hydr)oxide)] and Goethite [crystalline Fe-(hydr)oxide] as sorbent media in batch experiments. In particular, it was studied: i) the Langmuir isotherms of As(III) at pH 6.0; ii) arsenite sorption at different pHs; iii) arsenite sorption in presence of selected competing ligands, such as citrate (CIT), molybdate (MoO4), oxalate (OX), phosphate (PO4), selenite (SeO3), selenate (SeO4), and sulphate (SO4). 
Both Ferrihydrite and Goethite samples show a high affinity for As(III). However, the Ferrihydrite samples, characterized by a higher surface area (104.9 m2 g-1), were able to sorb much greater amounts of As(III) than the Goethite, with a lower surface area (42.3 m2 g-1). However, Ferrihydrite (Sm = 1245.1 mmol kg-1) adsorbed much more As(III) than Goethite (Sm = 327.9 mmol kg-1), essentially because of its greater surface area (178.2 vs. 42.3 m2 g-1) and, hence, lower degree of crystallinity, as well as to the greater affinity of As(III) for its surfaces. Probably, in our tests, Ferrihydrite adsorbed As(III) on their own external surfaces by forming a greater amounts of inner-sphere complexes vs. those formed by Goethite. 
The sorption of As(III) was lightly pH dependent. In fact, in the pH range of 4.0-9.0 the quantities of As(III) adsorbed resulted to be practically constant. The organic and inorganic ligands showed different capacities to compete with As(III) for the sorption sites of the Ferrihydrite and Goethite, according to the following increasing sequence: selenate&lt; sulphate &lt; oxalate &lt; citrate &lt; selenite ≈ molybdate&lt; phosphate on Ferrihydrite, and sulphate ≈ selenate&lt; oxalate &lt; citrate