Arsenate and arsenite adsorption in relation with chemical properties of alluvial and loess soils

Arsenic is one of the most toxic elements in the soil environment. Understanding of the arsenic adsorption chemistry is essential for evolving the extent of soil and groundwater contaminations. This research was conducted to determine the variation in adsorption behaviour of arsenite and arsenate with depth in different lithology soils. We sampled two parent materials at genetic horizons, and within a parent material, we selected two soils. Besides basic soil characterizations, a laboratory batch experiments were carried out to study the adsorption of arsenate and arsenite. Freundlich adsorption approaches were employed to investigate the adsorption of arsenate and arsenite in the soils. Freundlich isotherms fit arsenate and arsenite sorption data well with r values of 0.88–0.98 in most soils. Arsenate and arsenite adsorption varied with the soil properties, especially in clay composition and in the oxides of iron and aluminum. Arsenic adsorption parameters also varied with depth in parent materials, and loess derived soils had greater adsorption capacity as compared to alluvial soils in most of the adsorption parameters. This research concludes that the loess soils had higher arsenic adsorption capacity than the alluvial soils.


INTRODUCTION
Arsenic is one of the most toxic trace element present in soil and water.Naturally As occurs in rocks and mineral weathering and it can contaminate soil and groundwater. 1Anthropogenic activities, including the application of agricultural pesticides, industrial waste and land filling of sewage sludge add As to soil and cause As mineral weathering.Mineralization of organic matter and cyclic oxid-ARSENATE AND ARSENITE ADSORPTION PARAMETERS

Soil sampling and characterization
Soils were taken from the Pothwar Plateau, having different parent material, and we selected alluvium and loess parent material for the present study.The GPS location and USDA taxonomy of selected soils is given in Table I.Triplicate profile for each soil was dug and soil samples were taken from each of the genetic horizon.Samples were air dried and crushed to pass through a 2-mm sieve.Each sample was analyzed for texture, pH, calcium carbonate, cation exchange capacity (CEC), Na 2 S 2 O 4 -extractable Fe and Al and dissolved organic carbon (DOC).The soil particle size distribution was determined by the dispersion in (NaPO 3 ) 6 solution and the soil pH of saturated soil paste was measured.Soil CaCO 3 was determined by the acetic acid consumption. 19The DOC was extracted with K 2 SO 4 and determined by consumption of K 2 Cr 2 O 7. 20 Soil iron and aluminum oxides were dissolved in Na 2 S 2 O 4 and C 6 H 5 Na 3 O 7 and NaHCO 3 buffer solution.The concentrations of Fe and Al in the extracts were measured by ICP-MS. 21Cation exchange capacity was determined by saturating soil with an index cation. 22Amorphous iron and aluminum were determined by extracting soil in acidified ammonium oxalate solution in dark. 23

Batch experiment for arsenate and arsenite sorption
Sorption isotherms for As(V) and arsenite were constructed to estimate Fruendlich model parameters employing the batch sorption experiments. 24,25In triplicate, 3.0 g of soil was equilibrated with 30 mL 0.01 M KNO 3 solutions containing graded concentration of As(V) 0, 0.1, 2.5, 5, 8, 10, 15, 20, 25, 40 and 100 mg L -1 (Na 2 HAsO 4 ).The suspension was shaken for 48 h at room temperature and centrifuged for 20 min at 3000 rpm.The supernatant was filtered through 0.45 µm cellulose membrane and analyzed for total arsenic.Separately, three g soil was equilibrated with 30 ml of 0.01 M KNO 3 solution containing graded concentration of As(III) 0, 0.1, 0.5, 1, 3, 5, 7 and 8 mg L -1 from NaAsO 2 .The suspension was shaken for 48 h at room temperature and centrifuged for 20 min at 3000 rpm.The cellulose membrane of 0.45 µm was used to filter the supernatant and total arsenic was analyzed in the extract.Sorbed amount of As(V) and As(III) was calculated from the change in the solution phase concentration.
The adsorption isotherm was fitted to the Freundlich equation:  where x/m is the equilibrium concentration adsorbed by the soil (mg kg -1 ), c w is the equilibrium concentration in solution (mg L -1 ), β is an adsorption exponent related to adsorption intensity and K f is the Fruendlich adsorption coefficient (L kg -1 ).

Chemical analysis
Soil pH and pH change for CaCO 3 were determined by using Cole Parmer pH meter.For CEC analysis, NH 4 + determination was carried out using shimadzu UV-Vis spectrophotometer.Elemental analysis of Fe, Al, and total arsenic were determined by using Perkin Elmer's ICP-MS.

Statistical analysis
The variety of the adsorption parameters were ascribed to the soil parent material at different depths.The multivariate analysis was implemented using Proc General Linear Model in SAS version 9.4 (SAS Institute Inc., 2014). 27The parent material and soil (parent material) were class variable and the soil depths were multiple dependent variables.Stepwise multiple regression analysis was applied to correlate the adsorption parameters and the soil properties that determine adsorption of As(V) and As(III) in soils derived from loess and alluvium.

Soil characteristics
Soils varied in chemical and physical characteristics which are important for As(V) and As(III) adsorption (Table II).The soils were dominantly silt loam.Overall, the alluvium derived Kotli soil had larger clay content, followed by the Guliana, Mansehra and Rawal soils.Clay leaching and accumulation caused by the soil development processes was observed in Bt horizon of all the soils except for the Kotli, developed in clayey parent material.Most soils were non-calcareous with lower DOC.Rawal soil under forest conditions showed higher DOC content.Dissolved organic carbon varied in most soils with higher contents in surface horizons and decreased with depth, in all the soils due to the accumulation of organic material at the surface horizons.Most of the soils were non-calcareous except for the Rawal soil which had CaCO 3 in the range of 35 to 100 g kg -1 and increased with depth.The similar values for CaCO 3 were reported earlier. 28The CEC was in the range of 10 to 30 cmol+/kg.Higher CEC was observed in the Guliana soil with the increasing trend with depth and seemed to increase at Bt horizon level in all the soils.Amount of citrate bicarbonate dithionate extractable iron seems increased with depth in all the soils.Overall, the Guliana soil showed greater amount of Fe due to soil development process which results in the iron release from primary mineral.In the Rawal, Guliana and Mansehra soils high Fe content at Bt horizon was due to the translocation and the accumulation at Bt horizon level.Amount of citrate bicarbonate dithionate aluminum was greater in the loess derived soils, as compared to alluvium derived soils indicating the significant change in Al with the change in parent material. 29mount of citrate bicarbonate dithionate aluminum seems to be increased with the soil depth in all the soils.Loess derived Guliana and Mansehra were highly weathered soils, as supported by greater content of Fe and Al, compared to allu-(CC) 2017 SCS.All rights reserved._________________________________________________________________________________________________________________________ Available on line at www.shd.org.rs/JSCS/ARSENATE AND ARSENITE ADSORPTION PARAMETERS 947 vium derived Rawal and Kotli soils.Oxalate-extractable iron also varied with parent material.The loess derived Guliana and Mansehra soils had higher concentration of Fe compared to the alluvial Rawal and the Kotli soils.High Fe content was observed in soils at relatively higher stage of development and accumulated in Bt horizon, whereas in the Rawal and Kotli soils higher Fe was observed in the surface horizons due to less leaching. 30High amorphous iron contents in Mansehra soils may attribute to high rainfall which reduces the crystallinity while dry climate promote crystallinity. 31Oxalate-extractable aluminum was higher in soils at higher stage of development since the weathering processes result in an increase in hydrolytic breakdown and release of iron and aluminum from the primary minerals.Oxalate-extractable aluminum content also increased with an increase in the depth.Overall iron and aluminum oxides were present in greater amount as a result of weathering in Guliana and Mansehra soils.Total As content varied between 3.4 to 6.92 mg kg -1 in most soil samples.In almost all the soils total As content increase with depth, which may indicate subsurface accumulation.

Arsenate and arsenite sorption isotherm
The isotherms for As(V) and As(III) sorption depict variation in the shape of isotherms with the diversity in soil characteristics (Fig. 1).Overall, As(V) ads- orption was greater than As(III).Both As(V) and As(III) had fast rise in sorption, with a small increase in concentration in equilibrium solution.Several scientists reported a fast sorption initially and a moderate increase at the later part of the isotherm. 9,18Maximum sorption increase for As(V) was less than 150 mg kg -1 .The Bt horizons, especially in the case of the Mansehra and the Guliana soils, had faster adsorption rise compared to Ap and Bw horizons.In case of As(III), where the concentration maxima was ≈ 30 mg kg -1 generally, had greater rise in the adsorbed concentration in case of Bt horizons of all the soils.It appears that the clay content had a stronger role in the adsorption of both As(V) and As(III).Several studies indicated a strong correlation of clay content with the adsorption of As(V) and As(III). 32,33g. 1.Adsorption isotherms for arsenate and arsenite constructed between adsorbed concentrations at ordinate and solution concentration at abscissa in, indicating a fast initial rise especially in case of Bt horizon and the Bk horizon of Rawal had noteable low in adsorption.

Freundlich equation sorption parameters
Freundlich equation (Eq.( 1)) fit the isotherms with most of the r 2 > 0.95 (Fig. 2).Adsorption of As(V) and As(III) in these soils was well fitted by the Freundlich isotherm model. 34The maximum average adsorption intensity of As(III) (β) was lower than the adsorption intensity of As(V) in most soils, while the adsorption capacity (K f ) was greater for As(V) than As(III).It appears that As(V) had higher adsorption capacity than As(III), but with lesser binding intensity than most soils.The distribution of β and K f for As(V) and As(III) remained similar to the soil depth in both of the parent materials (Fig. 3), as the hypothesis of nonsignificant depth-PM interaction was accepted through MANOVA test criteria.Fruendlich adsorption parameters increased with soil depth in loess and alluvium.The loess derived soils had higher adsorption parameters (β and K f ) than alluvial soils at all soil depths.Variation in β and K f of As(V) and As(III) with the soil depth in each parent material was also remained similar (Table III).
(CC) 2017 SCS.All rights reserved._________________________________________________________________________________________________________________________ Available on line at www.shd.org.rs/JSCS/1)) fit for arsenate and arsenite isotherms in the selected soil horizons.Arsenate and arsenite adsorption parameters were calculated from the trendline.The adsorption intensity ranged from 1.45 to 2.50 and K f of As(V) ranged from 19 to 75 L kg -1 and.These results are in line with the findings of Roy et al., 35 Payne and Abdel-Fattah 36 and Zeng et al. 37 Arsenate β increased with depth in all the soils with the increase in clay content (r 0.48).As β is the intensity of adsorption, it is related to the abundance of metal oxides.It was evidenced that As(V) had high affinity for metal oxides from positive correlation of β with Fe d (r 0.77), Al d (r 0.80), Fe o (r 0.42) and Al o (r 0.71).Similarly, K f , increased with depth with increase in clay content as evidenced by the positive correlation with clay (r 0.46).The CEC is largely related to clay content 38 because there are more exchange sites for adsorption, so that As(V) K f values positively correlated with CEC (r 0.59).The K f had showed strong relation with iron (Fe d (r 0.83) and Fe o (r 0.50)) and aluminum (Al d (r 0.89) and Al o (r 0.87)) oxides as the increase in As(V) K f and in the metal oxides with increase in soil depth.Several studies reported the role or iron and aluminum oxides in the As(V) adsorption. 13,18Overall, As(V) β and K f were increased linearly with the increase in metal oxide contents.Loess derived Guliana and Mansehra soils, which are more weathered soils, had greater amount of clay in Bt horizons and ultimately higher iron and aluminum oxides, resulted in greater adsorption of As(V) when compared to alluvium derived Rawal and Kotli soils.The adsorption intensity, β, ranged from (CC) 2017 SCS.All rights reserved._________________________________________________________________________________________________________________________ Available on line at www.shd.org.rs/JSCS/ARSENATE AND ARSENITE ADSORPTION PARAMETERS 951 2.07 to 3.43 and K f ranged from 13 to 76 L kg -1 .The adsorption parameters vary widely with the change in soil properties especially due to the differences in iron oxides. 39The β values showed non-significant relation with the metal oxides indicating the weak binding of As(III) on the metal oxide surfaces.However, As(III) β was positively related with clay content (r 0.38) and CEC (r 0.36).The adsorption capacity of As(III) was higher in weathered horizons of the Guliana and the Mansehra soils while β values are greater in Bw/Bt horizons of all the soils compared to remaining horizons.The K f for As(III) increased with depth in all the soils except for the Kotli soil.Similar values for K f of As(III) were observed by Elkhatib et al. 39 in loamy soils.Arsenite K f showed similar behaviour as As(V) in the positive correlation with Fe d (r 0.77), Al d (r 0.87), Fe o (r 0.46) and Al o (r 0.82).The Bt horizons of the Guliana and the Mansehra soil had higher K f values due to the higher clay and metal oxides content.

Prediction of adsorption parameters
We selected the soil properties (independent parameters) by the stepwise regression maximizing r 2 and predicted the adsorption parameters of As(V) and As(III) obtained from the fitting of the isotherm to the Fruendlich equation.Soil was included as an independent parameter.Intercept differed with the soil type in case r 2 improves significantly, due to the addition of a soil type as a variable.The multiple regression equations have more significance in the understanding of processes rather than as predictive tools.The regression equations for As(V) and As(III) adsorption parameters are given in the Table IV.It is apparent that β for As(V) is positively related with Al d while negatively correlated with CEC, CaCO 3 , which confirms the observation of Raven et al. 18 that the adsorption intensity was more related to metal oxides.Freundlich coefficient related to the adsorption capacity K f in all soils was more related to Fe o and Al o as compared to Fe d and Al d . 40However, β for As(III) is suppressed by aluminum oxides (Al d and Al o ) whereas it is increased in the presence of iron oxides and CEC related to clay content. 38The regression equation for As(III) K f showed that the adsorption capacity of As(III) increases with the increase in Al d , whereas it was suppressed by the increase in CaCO 3 and CEC.

CONCLUSIONS
From our results, it can be concluded that As(V) adsorption was greater than As(III) in all the soils and varied with the soil properties, among which clay content and iron and aluminum oxides exhibited the most important influence on As(V) and As(III) adsorption.Freundlich isotherms fit As(V) and As(III) sorption data well with r 2 values of 0.88-0.98 in most soils.Loess soils had higher adsorption capacity than alluvial soils.Metal oxides and clay contents are the major predictor for As(V) and As(III) adsorption parameters.This study may help to understand the soil and groundwater contamination phenomena.

Fig. 2 .
Fig. 2. Freundlich equation (Eq.(1)) fit for arsenate and arsenite isotherms in the selected soil horizons.Arsenate and arsenite adsorption parameters were calculated from the trendline.

Fig. 3 .
Fig. 3. Distribution of adsorption parameters (β and K f ) of arsenate and arsenite in each parent material (mean = 6, and bars show standard error).

TABLE I .
Selected soils loess and alluvium parent materials 26

TABLE II .
Chemical properties of studied soils; CEC, cation exchange capacity; DOC, dissolved organic carbon; Fe d , Al d , dithionate extractable Fe and Al; Fe o , Al o , oxalate-extractable Fe and Al; mean of n = 3; standard deviation in the parentheses

TABLE III .
Fitted Freundlich sorption model parameters for arsenate and arsenite of each soil; adsorption intensity, β, and maximum adsorption capacity, K f , both in L kg -1 , were calculated from the respective regression equation.Values in parentheses are standard deviations

TABLE IV .
Regression equations for different properties estimate for each adsorption parameter (CC) 2017 SCS.All rights reserved._________________________________________________________________________________________________________________________ Available on line at www.shd.org.rs/JSCS/952 RUKH et al.