Introduction•For remediation of soils and purification of polluted waters, wastewaters, biosorbents might be considered as prospective groups of materials.•Amongst them peat have a special role due to low cost, biodegradability, high number of functional groups, well developed surface area and combination of hydrophilic/hydrophobic structural elements. •Hybrid biosorbent - natural, biomass based modified material, covered with another sorbent material, thus combining properties of both such as sorbent functionalities and surface properties.•As the “covering layer” both inorganic substances, mineral phases (iron oxohydroxides, oxyappatite) and organic polymers (using graft polymerization) were used.

•Environmental pollution with V and VI group metalloids (As, Sb, Te) – significant environmental problem. Polluted drinking water can cause important threats to the human health.

•Arsenic contamination of waters is a global problem. More than 40 million people are suffering from As contamination worldwide. Antimony and its compounds were considered as pollutants by USEPA as well as the EU. Antimony like arsenic is toxic and trivalent species are reported to be more toxic than pentavalent. Toxicity of tellurium may induce local environmental problems.•Sb as well as As is present in the environment as a result of natural and human activities. Predominant forms of As and Sb in natural waters are As (III), As (V) and Sb (III), Sb (V) accordingly, while tellurium mainly exists as Te (IV) or Te (VI).•Inorganic forms of As and Sb in natural waters and soils are intimately related to the presence of iron oxo(hydroxides) being metalloids strongly sorbed onto the surfaces of Fe oxides, forming inner- or outer-sphere surface complexes. Therefore Fe containing sorbents based on different natural materials have found widespread application in water treatment.

•The aim of this study is to investigate and compare Fe-modified biomaterial application possibilities for metalloid and phosphate sorption, thus reducing their concentration and obtain new effective and perspective sorbents.

Juris Burlakovs, Maris Klavins, Artis Robalds, Linda AnsoneDepartment of Environmental Science, University of Latvia, Raina blvd. 19, LV – 1586, Riga, Latvia

e-mail: juris@geo-it.lv, linda_ansone@inbox.lv

The present work investigates the sorption of metalloids (As, Sb, and Te) and phosphates – on Fe-modified biomaterials: peat, shingles, moss and straw using batch tests. Sorption was studied as a function of initial metalloid (or phosphate) concentration, pH, and temperature.

FT-IR spectra, SEM images, nitrogen adsorption isotherm measurements, as well as moisture content, organic substances content, and Fe2O3 analyses were used to characterize the obtained sorbents.

Results and DiscussionObtained sorption data have been correlated using Langmuir as well as Freundlich isotherm models. Based on correlation coefficients, metalloid sorption as sorbent using Fe-modified biomaterials better fitted to the Langmuir model with exception of Sb (V) as a sorbent using Fe-modified moss that fitted to Freundlich model.

Sorption capacity increases with a rise in temperature. Possible reason could be diffusion rate increase of adsorbate molecules as the temperature raise, as well as changes of sorbent pore size.

Linear Langmuir equation form was used to determine Langmuir isotherm constant (K) and thermodynamic parameters. Calculated thermodynamic parameters (ΔG°, ΔH˚, ∆S˚) suggest that sorption process has spontaneous nature and it is endothermic. The positive values of standard entropy show increasing randomness at the solid/liquid interface during the sorption of metalloid ions onto modified peat. Decrease in ∆G˚ with increasing temperature implies stronger sorption at higher temperatures.

Materials and Methods

Hybrid biosorbents for removal of pollutants and remediation

Sorbent modification with Fe compounds and product after graftpolymerization allow to enhance sorption capacity. The reason could be formation of metalloid-O-Fe bond for Fe-modified biosorbents. The highest sorption capacity has Fe-modified peat sorbing As (V), Sb (V) as well as Te (VI). Fe-modified moss and Fe-modified shingles could also be used in severely polluted waters, while Fe-modified straw and Fe-modified canes may be used in areas with low metalloid concentration.

T (K) K (L/mol) ∆G(kJ/mol)

As (V) Sb(V) Te (VI) As (V) Sb(V) Te (VI) 275 4802.0 10000 23000 -19.4 -21.06 -22.96 283 4628.1 14150 15600 -19.9 -22.49 -22.72 298 8838.2 20769.2 35500 -22.5 -24.63 -25.96 313 43331.9 32125 57800 -27.8 -27.00 -28.53

∆H (kJ/mol) ∆S (J/mol K)As (V) 41.43 214Sb(V) 21.98 156

Te (VI) 32.15 195

Sorbent: Fe-modified peat Sorbent: Fe-modified peat

Material modification with Fe compounds significantly enhanced sorption capacity, possibly due to metalloid-O-Fe bond forming. Fe-modified peat - the best sorbent for As (V), Sb (V) and Te (VI). The best sorption conditions are in the pH interval 3-6 for As (V) and Sb (V), while pH 3 – 9 is the most appropriate interval for Te

(VI) removal. The sorption capacity increases with increase in temperature for all studied metalloids and calculated thermodynamic

parameters suggest that sorption process has spontaneous nature and it is endothermic.

Conclusions

Sorbent LOI,

% Fe2O3, mg/g

Specific surface area (BET method),

m2/g

Point of zero charge pH (pH

zpc) mod. peat 74 424.8 44.16 4.7

mod. shingles 83 297.7 45.31 5.5 mod. canes 79 274.8 40.31 7.0 mod. moss 80 256.5 19.14 3.2 mod. straw 81 292.3 - 5.3 mod. sand 1.2 49.4 - -

Different metalloid concentrations (10-800 mg/L)

ETAAS or FAAS0.5 g Fe-modified sorbentStirred for 24 h, filtered

Analysis and calculations Results

Sorbent: Fe-modified peat

• It could be supposed that As (V) and Sb (V) as well as Te (VI) sorption onto Fe-modified biosorbents mainly occur considering electrostatic interaction between the ionic species of respective element and the charged surface groups of sorbent.

• pHzpc values of studied sorbents varies from 3.2 to 5.5. pHzpc of Fe-modified peat is 4.7, the surface of the sorbent is positively charged if pH < 4.7, and arsenic as well as antimony is negatively charged, and the sorption capacity is maximal. At the pH interval pH > 4.7, surface of the sorbents become negatively charged and interaction between active sorption sites on the sorbent and negatively charged ions decreases. The decrease of sorption capacity in basic solution could be due to the competition for sorption sites between metalloid anions and hydroxide ions.

• Speciation of metalloids in aqueous media is suggested to be one of the main factors that cause differences in metalloid removal.

• The best sorption conditions are in the pH interval 3 – 6 for As (V) and Sb (V), when negatively charged H2AsO4

-

and HAsO42- and Sb(OH)6

- are predominant forms in solution.

• pH has not significant impact Te (VI) removal at the studied conditions.

The maximum sorption capacity reached 11.10 mg P/g at initial P concentration of 500 mg/L.

• The sorption capacity of modified peat increased with the increase of the initial phosphate concentration for all temperatures studied (Figure 1).

• The steep rise of the isotherm curve close to its origin indicates high uptake values at low concentrations. 99 % of phosphate ions had been sorbed at initial concentration range 0.1 - 25 mg P/L, which is a range typical for wastewaters.

The positive values of standard entropy show increasing randomness at the solid/liquid interface during the sorption of metalloid ions onto modified peat

Taking into account the low cost and availability, the use of peat based sorbents may be considered as alternative to conventional wastewater treatment methods

Supplementary experiments should be carried out to evaluate the performance of developed sorbents in field scale systems – in constructed wetlands or in bed systems.

Characterization of Sorbents

SEM of iron modified peat

REMOVAL of PHOSPHATES

ACKNOWLEDGEMENTS - This work has been supported by the Latvia State Research Program within the project “Sustainable Use of Local Resources – New Products and Technologies” (NatRes).