Abubaker Edkymish - Mark- och vattenteknik - KTH

LEAD BINDING ONTO IRON OXIDES(FERRIHYDRITE) AS INFLUENCED BYALUMINIUM AND PHOSPHORUSAbubaker EdkymishJune 2009TRITA-LWR Degree ProjectISSN 1651-064XLWR-EX-09-15

Abubaker EdkymishTRITA LWR Degree Project© Abubaker Edkymish 2009Degree Project thesisWater System TechnologyDepartment of Land and Water Resources EngineeringRoyal Institute of Technology (KTH)SE-100 44 STOCKHOLM, Swedenii

Lead binding onto iron oxides (Ferrihydrite) as influenced by aluminium and phosphorusSAMMANFATTNINGBly är en vanlig markförorening från olika mänskliga verksamheter. I blyets biogeokemiskacykel anses adsorption till järnoxider vara en av de viktigaste processerna som styr blyetsrörlighet genom ekosystemet. I detta arbete studerades adsorptionen av bly (Pb 2+ ) tilljärnoxiden ferrihydrit (även benämnd HFO) vid olika pH-värden. Konkurrens frånaluminium (Al 3+ ) och interaktioner med fosfat (PO 43-) belystes. Resultaten användes för attoptimera en modell för blyadsorption till ferrihydrit. Här användes ytkomplexmodellen CD-MUSIC inom ramen för den geokemiska specieringsmodellen Visual MINTEQ. Enförbättrad adsorptionsmodell kan användas till att bättre kunna simulera blyets uppträdande imark- och vattensystem och detta kan t.ex. utnyttjas i riskbedömningar.På laboratoriet syntetiserades så kallad 2-linjeferrihydrit som hölls i en suspension där 0.01 Mnatriumnitrat (NaNO 3) utgjorde bakgrundselektrolyt. I en serie prover varierades det slutligapH-värdet genom tillsats av syra (HNO 3) eller bas (NaOH), och adsorption av bly studeradesvid flera olika koncentrationskvoter bly/järn i suspensionen. En konstant jämviktningstid på24 h användes i samtliga försök.Resultaten visade en stark blyadsorption, särskilt vid högt pH, som dock varierade kraftigtsom funktion av koncentrationskvoten bly/järn i suspensionen. Konkurrens från Al 3+ visadesig vara av mindre betydelse. Däremot var påverkan från PO 4-3 avsevärd; närvaro av PO 4-3förstärkte blyadsorptionen betydligt. Som ett exempel kan nämnas resultaten från systemsom innehöll 28,2 µmol/l Pb 2+ som tillsatts till 0,27 g/l ferrihydrit. Här uppgickadsorptionen av Pb 2+ vid pH 3.1 till 14,9% av den tillsatta mängden när ingen PO 4-3 tillsatts.I närvaro av en totalkoncentration av 600 µmol/l PO 4-3 uppgick adsorptionen av Pb 2+ vidpH 3.1 till 86,9%, det vill säga en mycket kraftig förstärkning. Utfällning av blyfosfatmineralförkastades som en trolig förklaring beroende på undermättnad gentemot kända sådanamineral i jämviktsextrakten. I ytkomplexmodellen kunde däremot effekten från PO 4-3simuleras korrekt förutsatt att Pb 2+ och PO 4-3 bildar ett ternärt komplex med ferrihydrit.Vidare hade bly (Pb 2+ ) ingen stor effekt på adsorptionen av Al 3+ och PO 4-3 till ferrihydrit idet koncentrationsintervall som studerades.En viktig slutsats från arbetet är att tillsats av järnoxidinnehållande material till leda tillminskade koncentrationer av Pb 2+ i vatten, särskilt i miljöer med höga koncentrationer PO 4-3.Nyckelord: adsorption; ferrihydrit (HFO); tungmetaller; modell; ytkomplexmodell(CD-MUSIC); Visual MINTEQ.iii

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Lead binding onto iron oxides (Ferrihydrite) as influenced by aluminium and phosphorusACKNOWLEDGEMENTSMy sincere thanks to Jon Petter Gustafsson for his valuable guidance of my work, superbknowledge of laboratory procedures, pertinent literature, and how to phrase a scientific documenthave been invaluable for this thesis, also my thanks to Monica Löwén for her assistanceat some stages in laboratory work, and finally great thanks to my wife Abu-tweraat L.for her supporting during my study.v

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Lead binding onto iron oxides (Ferrihydrite) as influenced by aluminium and phosphorusTABLE OF CONTENTSammanfattningiiiAcknowledgementsvTable of ContentviiAbstract 1Introduction 1Lead in the environment 2Adsorption processes 2Iron Oxides 3Ferrihydrite (HFO) 3Adsorption models 4Lead (Pb 2+ ) adsorption mechanism onto HFO 5Phosphate (PO 4-3) adsorption mechanism onto HFO 5Materials and Methods 6Stock solutions 6Batch experiments with ferrihydrite 6Analytical procedures 6Geochemical Modeling Software 7Results 8Optimizing the model: 8Lead: 8Aluminium: 8Phosphate: 8Investigation of interaction effects: 8Influence of Al 3+ on Adsorption of Pb 2+ onto HFO: 8Influence of PO 3- 4 on adsorption of Pb 2+ onto HFO: 8Influence of Pb 2+ on the adsorption of Al 3+ onto HFO: 8Influence of Pb 2+ on the adsorption of PO 3- 4 onto HFO: 9DISCUSSION 24Conclusion 26References 27vii

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Lead binding onto iron oxides (Ferrihydrite) as influenced by aluminium and phosphorusABSTRACTIn the biogeochemical cycle of lead, sorption to iron oxides is considered to be an importantprocess that restricts lead mobility. In this study lead (Pb 2+ ) sorption was studied at differentpH values to quantify the competitive effect of aluminum (Al 3+ ) and phosphate (PO 43-) ions,and to drive a model for the adsorption of Pb 2+ on an amorphous iron oxide ferrihydrite(HFO) by using a surface complexation model (CD-MUSIC) within the Visual MINTEQsoftware system. This would help to predict and manage the fate of toxic metal Pb 2+ in aquaticenvironments and in soils. 2-line ferrihydrite was synthesized to obtain suspensions inNaNO 3 as background with an ionic strength 0.01 M over a range of pH values. The impactof Al 3+ on the adsorption of Pb 2+ was not significant, while the effect of PO 4-3 was substantial.As an example, in systems with 28.2 µmol/l Pb 2+ added to 0.27 g/l ferrihydrite, the Pb 2+adsorption at pH 3.1 was 14.9% when no PO 4-3 had been added. In the presence of 600µmol/l PO 4-3 however, the Pb 2+ adsorption at pH 3.1 was 86.9%, i.e. much stronger. In thesurface complexation model, this effect could be described if Pb 2+ and PO 4-3 forms a ternarycomplex with ferrihydrite. In the concentration range examined, lead (Pb 2+ ) did not have anysignificant influence on the adsorption of Al 3+ and PO 4-3 onto ferrihydrite. Thus, applicationof iron oxide-containing materials can lead to a decrease of Pb 2+ concentrations in the aquaticenvironment, especially in environments high in PO 4-3.Key words: Adsorption; Ferrihydrite (HFO); heavy metals; Modelling; Surface complexationmodel (CD-MUSIC); Visual MINTEQ.INTRODUCTIONOur universal environment consists of several metals, which played a vitalfunction in human development. Many of them are of concern forthe reason that of their toxic properties; some of them are also necessaryfor survival. Metals are non-degradable, but may sometimes accumulatein certain areas due to the growing demands of new culture and civilization.So, there should be a balance between the human health and metalsconcentration in our environment. Soils are often the main sinks foranthropogenic sources of this metal (Nriagu 1978). Furthermore, agriculturaluses, landfill leachate, urban runoff, industrial release, and miningactivities are some major sources of heavy metals. Aqueous contaminationsby heavy metals are still a global environmental problem, due totheir toxicity effects and accumulation through the food chain. Of theseheavy metals, lead Pb 2+ is one of the most investigated metals, due to itshigh danger to the aquatic environment (Volesky 2001; Rama et al 2002).If heavy metals remains in dissolved form, or are incompletely fixed insoils or sediments toxic effects to humans, animals and surrounded environmentmay occur. The presence of heavy metals in polluted and contaminatedenvironment were documented or reported in several studies(Allen et al 1995). The fate of trace metals in an aquatic environment dependscrucially on sorption reactions at organic or inorganic particle surfaces,which are considered to be very important environmentally, due tothe strong role these processes have on both transport of metals andtheir toxicity. Several research projects have evaluated the adsorption ofmetal cations on surfaces of oxides such as iron oxides (Hiemstra & VanRiemsdijk 2002; Gustafsson 2003; Bradle 2004). Elimination or reductionof heavy metal ions from aquatic environments by adsorptionprocess are considered to be one of the main effective methods in thisfield. Thus, the quality of this process depends on the magnitude or abilityof the adsorbent to adsorb metal ions from their medium on its surfaces(Yuan et al 1999).1

Abubaker EdkymishTRITA LWR Degree ProjectThe aims of this research were: To investigate the possibility to model the adsorption of Pb 2+ on anamorphous iron oxide ferrihydrite (HFO) in a consistent way by usingthe CD-MUSIC surface complexation model, so that the fate oftoxic metals, such as Pb 2+ can be properly evaluated for aquatic andsoil environments. To quantify the competitive effect of aluminum and phosphate ionsat different pH values on the binding of lead (Pb 2 +) to ferrihydrite, tooptimize the conditions for maximum adsorption.Lead in the environment:The trace metal lead (Pb 2+ ) is a toxic natural component that occurs inalmost all geomedia (Kabata & Pendias 2001). Lead (Pb 2+ ) concentrationscan be elevated as a result of repeated inputs such as sewage andwastes disposal, farming sources, combustion of fossil fuel, miningprocess and electronic and metallurgical industries (Basta & Ryan 2005).Release of Pb 2+ into the broad environment occurred as a result of usingleaded gasoline, paint containing Pb, lead in pipeline systems of water,and smelter. However, the atmospheric lead emissions have been decreaseddue to removal of Pb from gasoline and limitations on Pb releasefrom point sources disposal. The main properties of elemental Pb are:shiny metallic, good electrical and heat conductor has a comparably lowmelting point, high density, and as a result of its corrosion resistance it isstill used in a variety of applications (Jose & Jose 2006). Environmentalfactors such as, for example, pH, temperature, salinity, and humic contentaffect an aquatic organisms uptake of Pb from water and sediment.Lead toxicity occurs when excessive concentrations of metal are absorbedin the lungs and/or stomach. The poisoning occurs little by little,due to regular Pb absorption within tissue, kidney, liver and bone, aftercontinuous exposure. Furthermore, kids are more sensitive than adultsdue to their ability to absorb Pb more simply and quickly (NAS/NRC1993). Pb is not metabolized, and the excretion of Pb is low and occursprimarily through the urinary tract. Absorbed Pb is eliminated primarilyvia the kidney in the urine (about 76%) and to a lesser extent by the gastrointestinaltract (about 16%) through biliary secretion, other routes forelimination (hair, nails, sweat and skin) account for approximately 8%.Lead (Pb) is also excreted in milk in concentrations of up to 12 µg/l. Ingeneral however, lead (Pb) is excreted very slowly from the body with abiological half-life estimated at 10 years, thus facilitating accumulation inthe body (Axelson & Hogstedt 1994).Adsorption processes:Adsorption is the accumulation of a chemical species or material at aninterface (often enabled by variable charge developed on the surfaces).The pH value is generally considered to be the most important parametergoverning adsorption. The negative charge that develops on variablechargesurface at high pH attracts cations, whereas the reverse phenomenonhappens at low pH (Sposito 2004). As mentioned before, adsorptionis considered to be an important in several geochemical processes,for instance contaminant elimination from the aquatic environment(Giblin 1980). Adsorption may be one of the few processes are able toeliminate trace metals in significant quantities from aquatic media. Otheravailable processes such as chemical precipitation, reverse osmosis andother methods are ineffective when contaminants are present in low ortrace quantity (Huang & Morehart 1991). A wide literature exists on theuse of adsorption techniques for removal of contaminants (inorganic or2

Lead binding onto iron oxides (Ferrihydrite) as influenced by aluminium and phosphorusorganic) by means of constituents either purified or synthetic, for instancemineral oxides, clay, and humic acids (Laird et al 1992; Zachara etal 1993; Cox et al 1995; Sarker et al 2000; Hundal et al 2001; Goldberg et al2007). The fate of trace metals in our environment are chiefly managedwith sorption reactions by active soil colloidal constituents (metal oxides,clay minerals, and organic matter) which are considered to be importantheavy metal sorbents due to their ability to form inner-sphere complexesthrough surface reactive groups and/or due to their cation-exchange capacity(Jelinek et al 1999; Weng et al 2001). Heavy metals are often morestrongly adsorbed by oxide minerals than by other phases in the naturalaquatic environment (Jenne 1977).Iron Oxides:Iron oxides are often the most abundant metallic oxides in soils, whichoccur in the form as oxy-hydroxides, and hydrated oxides of iron, andthey are commonly present as very small particles in one or more of theirmineral forms and aggregates in different climatic regions (Schwertmann& Taylor 1989). Many iron oxides have been identified (Table 1) ascommon abundant metallic oxides in the soil and colloids in aquatic environments.Thus, iron oxides play an important role for the mobilityand bio-availability of heavy metals. Iron oxides are considered to be thebest candidates for toxic heavy metals remediation in the soil due to theirabundance in the soil (Basta & Ryan 2005). The reactive surface of ironoxides interacts strongly with the soil solution (Schwertmann & Taylor1989). The reactivity and surface areas make iron oxides good sorbentsof several inorganic cations such as lead (Pb), cobalt (Co), zinc (Zn),copper (Cu), and nickel (Ni) (Sparks & Donald 2003). So, Fe oxides takepart in the transformations of nutrients and pollutants in soils and relatedenvironments (Dzombak & Morel 1990). Properties such as, surfacecharge, surface area, surface chemical composition, structure, surfaceelectronic properties, surface morphology and surface geometry aresome of the most important determinants of reactivity of the sorbentsand they govern their behavior in the associated environments.Ferrihydrite (HFO):Ferrihydrite, which is also referred to as HFO (hydrous ferric oxide) isthe least crystalline of all iron oxides, poorly ordered, and it is a characteristiccomponent of young Fe-oxide accumulations. It is a familiar elementof temperate area soils and sediments. However, it is not easy toseparate or categorize in situ (Schwertmann et al 1982). It is an amorphousor poorly crystalline ferric oxide, it is structurally similar to hematitebut a number of oxygen atoms (O) are replaced by hydroxide groups(OH) or water molecules (H 2O) (Towe & Bradley 1967). A comparisonresults of the macroscopic tests shows that amorphous oxides have largersorption capacities for metal contaminants than crystalline oxides suchas goethite, due to their higher specific surface area and larger content ofreactive surface groups. For that reason, HFO may be highly efficient asa metal sorbent in nature and might be the best sink for certain contaminants.It forms upon rapid hydrolysis of ferric iron solutions at 20 to 30°C. The ferrihydrite (HFO) specific surface area varies according to themeasurement method (Table 2) between 200 and 800 m 2 /g, which is alsogreater than goethite (Dzombak & Morel 1990). While all the other ironoxides have long-range order structures, ferrihydrite has short-range orderand a discontinuous layered structure, which gives the answer to itshigh surface area and sorption capacity (Spadini et al 1994).3

Abubaker EdkymishTRITA LWR Degree ProjectTable 1: The Major Iron Oxides and OxyhydroxidesOxyhydroxidesOxidesFormula Mineral Formula Mineralα-FeOOH Goethite Fe 5HO . 8 4H 2O Ferrihydriteβ-FeOOH Akaganeite α-Fe 2O 3 Hematiteγ-FeOOH Lepidocrocite γ-Fe 2O 3 Maghemiteδ'-FeOOH Feroxyhyte Fe 3O 4 MagnetiteTable 2: specific surface area for HFO. (Dzombak & Morel 1990)A: [m 2 /g] Method Source159 BET, N₂(g) (Avotins 1975)700 Negative absorption of Mg 2+ at pH 5 (Avotins 1975)159 BET, N₂(g) (Crosby et al 1983)234 BET, N₂(g) (Crosby et al 1983)300±50 BET, H₂O (Davies et al 1984)590 Ethylene glycol adsorption (Pyman & Posner 1978)215 BET, N₂(g) (Yates et al 1977)720 Calculated from Γ PO4(max), assuming 0.5 –nmdiameter for PO 4257 Continuous flow method (P-E sorptometer212D); gas adsorption, presumably.(Anderson& Malotky 1979)(Parfitt et al 1975)Adsorption models:Adsorption processes may be described by different models such as:1. Empirical models: for adsorption data descriptions not includingtheoretical foundation, like Freundlich adsorption isotherm, Langmuiradsorption isotherm and distribution coefficient.2. Chemical models: an essential engineering apparatus for the reasonthat to predict the removal of contaminants, such as lead (Pb) andzinc (Zn), from aquatic environment in vary sorbent concentrationsand environmental conditions.3. Surface complexation models: surface complexation models (SCMs)based on thermodynamics, and they have been developed to predicttrace metal sorption onto mineral oxides, speciation and partitioningin aqueous systems over a broad range of conditions. The modelsconceptualize adsorption reactions surface sites as being closely relatedto those of aqueous-phase ligands (McKinley et al 1995). Themodels may account for the fact that the surfaces of oxides of manyseparate sites with diverse binding strengths (Benjamin & Leckie1981a). Computer programs used to optimize parameters for the surfacecomplexation models. Chemical models planned to computethermodynamic properties values mathematically, like constant capacitancemodel (CCM), diffuse layer model (DLM), triple layer model(TLM), MICROQL and FITEQL (Westall 1982).4. Equilibrium speciation models: they consist of the surface complexationmodels: MINTEQ, SOILCHEM, and HYDRAQL (Papelis &Leckie 1988; Gustafsson 2008).5. Transport models: containing surface complexation models includeTRANQL and HYDROGEOCHEM (Cederberg et al 1985; Yeh &Tripathi 1990).4

Abubaker EdkymishTRITA LWR Degree ProjectThe structure of phosphate surface complexes on Fe oxides was postulatedas a monodentate and/or as a binuclear complex:Monodentate surface complexbinuclear surface complexIn case of the monodentate surface complex one PO 4-3 exchanges withone OH - ion. Therefore, the molar ratio of the released hydroxyl ions tothe adsorbed phosphate ions is 1/1, but if the binuclear complex onePO 4-3 replaces two OH - ions. In this case, the molar ratio of the releasedOH - ions to the adsorbed PO 4-3 ions should be 2/1 (Rajan 1975).MATERIALS AND METHODSStock solutions:High-purity distilled, de-ionized water was used for preparing solutions.In addition, stock solutions of NaNO 3 0.03 mol/l, Al(NO 3) 3 9.0 & 0.9mmol/l, Pb(NO 3) 2 0.9 &0.09 mmol/l, NaOH 0.03 & 0.01 mol/l, HNO 30.01 mol/l, NaH 2PO 4 1.8 mmol/l, and HFO 35 mmol/l suspended in0.012 mol/l NaNO 3 were used. All chemicals used in the experimentswere reagent grade.Batch experiments with ferrihydrite:2-line ferrihydrite (Swedlund & Webster 1999) was synthesized by addinga solution containing 36 mmol/l Fe(NO 3) 3 and 12 mmol/l NaNO 3to pH 8.0 through dropwise addition of 4 mol/l NaOH. Before thebatch experiments, the ferrihydrite suspensions where back-titrated topH 4.6 with 0.1 mol/l HNO 3 and vigorously shaken for 15-30 min.Batch experiment suspensions were prepared by mixing an amount offerrihydrite suspension with stock solutions of NaNO 3 and appropriatesalts (as: Pb(NO 3) 2, Al(NO 3) 3, and NaH 2PO 4) to obtain suspensions withan ionic strength of 0.01 M. By using HNO 3 and NaOH a range of differentpH values were obtained. In the single-sorbate systems only onesalt was added at concentrations of: 0.3 & 3.0 µM Pb, 30 & 300 µM Al,60 & 600 µM PO 4; where sorption was studied at two different ferrihydriteconcentrations at 0.3 & 3.0 mM total Fe. In the binary –competitive- systems, the ferrihydrite concentrations and the previoussalts were the same. The samples were equilibrated in 40 ml polypropylenecentrifuge tubes. After 24 h of equilibration in tightly sealed polypropylenebottles on an end-over-end shaker at room temperature(about 21 °C), the samples were centrifuged for 20 min at about 4000rpm, then samples were allowed to stand about 10 minutes to returnback to laboratory temperature, and filtered by using 0.2-µm AcrodiscPF single-use filters.Analytical procedures:The pH was measured on the unfiltered sample, using a Radiometercombination electrode. Before metal analysis, the filtered samples wereacidified with 0.5% HNO 3 and analyzed to determination of dissolved6

Lead binding onto iron oxides (Ferrihydrite) as influenced by aluminium and phosphorusPb and Al by ICP-MS using a Perkin-Elmer ELAN 600 instrument. Theconcentration of PO 4 was analyzed on filtered un-acidified samples usingan Aquatic Analyzer (AQUATEC ver. 1.02 5400) employing spectrophotometricaldetection of dissolved PO 4 at 690 nm with the molybdate method.Geochemical Modeling Software:Equilibrium constants were optimized using the Visual MINTEQ ver.2.60 chemical equilibrium model for the calculation of metal speciation,solubility equilibrium etc. for natural waters with specific parameters(Table 3) (Gustafsson 2008). All data were treated by this software to calibrateand compare observed data with modeled data. The Three-PlaneCD-MUSIC model was used, using the parameterization of ferrihydriteas reported by Gustafsson (Gustafsson 2001; Gustafsson et al 2009). Inthis model it was assumed that the Stern layer capacitances and electrolyteion-pair reaction constants were equal to those of goethite, but aspecific surface area of 750 m 2 g -1 was assumed to be valid for ferrihydrite.Model parameters and surface complexation constants used in thiswork are tabulated (Table 3 & Table 4).Table 3: Model parameters for surface charging of ferrihydriteaFerrihydriteA / m 2 g -1750 aNs / sites nm -2 6.3% high-affinity sites 1C 1 / F m -2C 2 / F m -21 b0.74 ba: From Gustafsson (Gustafsson 2001).b: Assumed to be equal to those of goethite (Hiemstra & van Riemsdijk 2006).Table 4: Surface complexation reaction. Data Source A - Dzombak & Morel 1990;source B -Hiemstra & van Riemsdijk 2006; source C – Gustafsson et al 2009.Reaction (Δ Z0, Δ Z1, Δ Z2) a log K b SourceFeOH ½- + H+ ↔ FeOH 2½+(1, 0, 0) 8.1 AFeOH ½- + Na + ↔ FeOHNa ½+ (0, 1, 0) -0.6 BFeOH ½- + H + + NO 3 - ↔ FeOH 2NO 3½-(1, -1, 0) 7.42 BFeOH ½- + H + + Cl - ↔ FeOH 2Cl ½- (1, -1, 0) 7.65 B2FeOH ½- + Pb 2+ + H 2O ↔ (FeOH) 2PbOH 0 + H + (0.1 ,0.9, 0) 1.28, 4.44 C2FeOH½- + 2H + + PO 4 3- ↔ Fe 2O 2PO 2 2- + 2H (0.46, -1.46, 0) 27.00 C2FeOH ½- + 3H + + PO 4 3- ↔ Fe 2O 2POOH - + 2H 2O (0.63, -0.63, 0) 32.44 CFeOH ½- + 3H + + PO 4 3- ↔ FeOPO 3H 2 ½- + H 2O (0.5, -0.5, 0) 30.22 C3FeOH ½- + Al 3+ + 2H 2O ↔ (FeOH) 3Al(OH) 2 ½- + 2H + (0.95, 0.05, 0) -0.13 C2FeOH ½- + 2H + + Pb 2+ + PO 4 3- ↔ (FeOH) 2PO 2Pb 0 + 2H 2 (0.6, 0.4, 0) 33.5, 36.49 Ca: The change of charge in the o-, b- and d-planes respectively.b: Two numbers indicate binding to high-affinity sites and low-affinity sites, respectively.7

Abubaker EdkymishTRITA LWR Degree ProjectRESULTSOptimizing the model:To optimize the CD-MUSIC model, additions of Pb 2+ , PO 4-3 and Al 3+were added separately to systems with different sorbate to sorbent ratios.Lead:Adsorption of Pb 2+ (Fig. 1) was studied at ratios of Pb (T)/Fe (9.4*10 -4 ,9.4*10 -3 and 9.4*10 -2 ) and at different pH values (Table 5). All systemsshowed a strong pH dependence, but the pH at 50% adsorption variedconsiderably (Table 5), showing that the heterogeneity of Pb binding wasstrong. The model could describe the data using two different complexes,a high-affinity and a low-affinity surface complex, where the formercould be formed on only a subset (1%) of the sites (Table 3 & 4).Aluminium:The adsorption percentage of Al 3+ was studied at ratios of Al (T)/Fe(1.2*10 -3 , 1.0*10 -2 and 1.0*10 -1 ) and at different pH values (Table 6).Again the pH dependence of adsorption was very strong. In contrast tothe Pb systems, the pH at 50% adsorption was rather similar. The use ofone tridentate complex in the model was sufficient to correctly reproducethe data (Table 4 & Fig. 2).Phosphate:The adsorption percentage of PO 4-3 was studied at a ratio of PO 4-3 (T)/Fe(2.0*10 -1 ) and at different pH values (Table 7). In this case the adsorptionwas weakly pH-dependent, with the strongest adsorption occurringat low pH. Three surface complexes were needed to describe the datacorrectly (Table 4 & Fig. 3).Investigation of interaction effects:In this section the capacity of the model to predict the adsorption in binarysystem (lead-aluminum or lead-phosphate) was investigated, and ifnecessary new complexes were included.Influence of Al 3+ on Adsorption of Pb 2+ onto HFO:Three different ratios of Pb (T)/Fe were studied to determine the rate ofadsorption of Pb in the presence of different concentrations of Al 3+ . Theresults are summarized in (Table 8) and (Fig. 4, 5, 6, 7, 8 & 9).Influence of PO 43- on adsorption of Pb 2+ onto HFO:Three different ratios of Pb (T)/Fe were studied to determine the adsorptionof Pb 2+ in the presence of two different concentrations of PO 43-. Itwas clear from the results that the adsorption of Pb 2+ increase in thepresence of PO 4 (Table 9) and (Fig. 10, 11, 12, 13, 14 & 15).Influence of Pb 2+ on the adsorption of Al 3+ onto HFO:The adsorption of Al +3 onto ferrihydrite surface logically increases withincreasing pH, partly because of increased negative charge on the surfaceduring increasing of pH, and the resultant decrease in repulsion and theincrease in attraction. Three multiple cases (Table 10) were studied withone Al (T)/Fe ratio (1.0*10 -1 ), two different concentrations of total Fe,two different concentrations of Al +3 and two different concentrations ofPb +2 . The result was predicted by the model (Fig. 16, 17& 18). When theinfluence of lead (Pb) was tested on the adsorption of aluminum ontoferrihydrite we did not find any significant difference (Fig.19, 20 & 21).8

Lead binding onto iron oxides (Ferrihydrite) as influenced by aluminium and phosphorusInfluence of Pb 2+ on the adsorption of PO 43- onto HFO:In examining the impact of lead (Pb) on the adsorption of phosphorusonto ferrihydrite, the results which obtained from laboratory experimentsfrom three different cases with equal ratios of PO 4(T)/Fe. The resultsare summarized (Table 11) and presented (Fig. 22, 23, 24, 25, 26& 27).Table 5: Aqueous solution concentrations and results:PbpHTotalFemMInitialconc.µMadsorbedMin.%Max.%Min.AdsorbedPbMax.AdsorbedPb50%AdsorbedPbRatioofPb (T)/Fe3.0 2.82 14.9 97.27 3.06 4.85 3.65 9.4*10 -43.0 28.2 4.09 99.41 3.07 5.92 4.25 9.4*10 -30.3 28.2 4.2 99.98 2.97 9.47 5.5 9.4*10 -2Figure 1: Adsorption of Pb onto HFO in single sorbate system. Points are measureddata, and the lines are modeled data by the CD-MUSIC surface complexation model.Table 6: Aqueous solution concentrations of Al in single system:TotalFemMInitialconc.µMAladsorbedMin.%Max.%Min.AdsorbedAlpHMax.AdsorbedAl50%AdsorbedAlRatioofAl (T)/Fe3.0 3.65 1.33 94.84 3.07 4.76 4.1 1.2*10 -33.0 30 0.55 98.50 3.05 4.87 4.12 1.0*10 -20.3 30 5.02 98.60 3.01 6.9 4.6 1.0*10 -13.0 300 0.83 99.79 3.05 5.65 4.3 1.0*10 -19

Abubaker EdkymishTRITA LWR Degree ProjectFigure 2: Adsorption of Al onto HFO in single sorbate system. Points are measureddata, and the lines are modeled data by the CD-MUSIC surface complexation model.Table 7: Aqueous solution concentrations and resultsTotalFemMInitialconc.mMPO 4Min.%adsorbedMax.%Min.AdsorbedPO4pHMax.AdsorbedPO450%AdsorbedPO4RatioofPO 4(T)/Fe0.3 0.06 8.73 94.3 8.47 3.03 6.6 2.0*10 -13.0 0.6 46.6 99.42 7.7 3.21 7.4 2.0*10 -1Figure 3: Adsorption of PO 4 onto HFO in single sorbate system. Points are measureddata, and the lines are modeled data by the CD-MUSIC surface complexation model.10

Lead binding onto iron oxides (Ferrihydrite) as influenced by aluminium and phosphorusTable 8: Aqueous solution concentrations in Pb-Al binary system:PbpHTotal Initial adsorbedAlMin. Max. 50% RatioFeConc.of Fig.conc. Min. Max. Adsorbed Adsorbed AdsorbedmM µMmM% %Pb Pb Pb Pb(T)/Fe3.0 2.82 19.53 99.87 0.3 3.06 7.89 3.7 9.4*10 -4 43.0 28.2 6.49 99.99 0.3 3.07 7.47 4.65 9.4*10 -3 50.3 28.2 0.48 11.57 0.03 3.5 4.67 - 9.4*10 -2 6Figure 4: Adsorption of lead (2.82 µM added) to HFO (3 mM) in the presence ofaluminium (0.3 mM) binary-sorbate system. Points are measured data, and the line ismodeled data by the CD-MUSIC surface complexation model using constants optimizedfrom single sorbate system.Figure 5: Adsorption of lead (28.2 µM added) to HFO (3 mM) in the presence ofaluminium (0.3 mM) binary-sorbate system. Points are measured data, and the line ismodeled data by the CD-MUSIC surface complexation model using constants optimizedfrom the single sorbate system.11

Abubaker EdkymishTRITA LWR Degree ProjectFigure 6: Adsorption of lead (28.2 µM added) to HFO (0.3 mM) in the presence ofaluminium (0.03 mM) binary-sorbate system. Points are measured data, and line ismodeled data by the CD-MUSIC surface complexation model using constants optimizedfrom the single sorbate system.Figure 7: Comparison between Pb 2+ single adsorption at a Pb(T)/Fe ratio of 9.4*10 -4(blue rhombus marks), and binary adsorption (red triangle marks) in the presence of0.3 mM Al 3+ .12

Lead binding onto iron oxides (Ferrihydrite) as influenced by aluminium and phosphorusFigure 8: Comparison between Pb 2+ single adsorption at a Pb (T)/Fe ratio of 9.4*10 -3(blue rhombus marks), and binary adsorption (red triangle marks) in the presence of0.3 mM Al 3+ .Figure 9: Comparison between Pb 2+ single adsorption at a Pb (T)/Fe ratio of 9.4*10 -2(blue rhombus marks), and binary adsorption (red triangle marks) in the presence of0.03 mM Al 3+ .13

Abubaker EdkymishTRITA LWR Degree ProjectTable 9: Aqueous solution concentrations in Pb-PO 4 binary systems:TotalFemMInitialconc.µMPbadsorbed%Min.%Max.PO 4conc.mMMin.AdsorbedPbpHMax.AdsorbedPb50%AdsorbedPbRatioofPb(T)/Fe3.0 2.82 % 87.61 % 100 0.6 3.14 5.85 - 9.4*10-4 103.0 28.2 57.21 100 0.6 3.12 6.34 - 9.4*10-3 110.3 28.2 3.94 99.97 0.06 2.95 7.55 4.35 9.4*10-2 12Fig.Figure 10: Adsorption of lead (2.82 µM added) to HFO (3.0 mM) in the presence ofphosphate (0.6 mM) binary-sorbate system. Points are measured data, and the line ismodeled data by the CD-MUSIC surface complexation model using constants optimizedfrom single sorbate systems.14

Lead binding onto iron oxides (Ferrihydrite) as influenced by aluminium and phosphorusFigure 11: Adsorption of lead (28.2 µM) added to HFO (3.0 mM) in the presence ofphosphate (0.6 mM). Points are measured data, and the line is modeled data by theCD-MUSIC surface complexation model using constants optimized from single sorbatesystems and assuming the presence of Pb-PO 4 ternary complex.Figure 12: Adsorption of lead (28.2 µM) added to HFO (0.3 mM) in the presence ofphosphate (0.06 mM). Points are measured data, and the line is modeled data by theCD-MUSIC surface complexation model using constants optimized from single sorbatesystems and assuming the presence of Pb-PO 4 ternary complex.15

Abubaker EdkymishTRITA LWR Degree ProjectFigure 13: Comparison between Pb 2+ single adsorption at a Pb (T)/Fe ratio of 9.4*10 -4(blue rhombus marks), and binary adsorption (red triangle marks) in the presence of0.6 mM PO 4-3.Figure 14: Comparison between Pb 2+ single adsorption at a Pb (T)/Fe ratio of 9.4*10 -3(blue rhombus marks), and binary adsorption (red triangle marks) in the presence of0.6 mM PO 4-3.16

Lead binding onto iron oxides (Ferrihydrite) as influenced by aluminium and phosphorusFigure 15: Comparison between Pb 2+ single adsorption at a Pb (T)/Fe ratio of 9.4*10 -2(blue rhombus marks), and binary adsorption (red triangle marks) in the presence of0.06 mM PO 4-3.Table 10: Aqueous solution concentration in Al-Pb binary systems:TotalFemMInitialconc.µMAlMin.%adsorbedMax.%Pbinitialconc.µMatMin.AdsorbedAlpHatMax.AdsorbedAlat50%AdsorbedAlRatioofAl (T)/Fe3.0 300 8.03 99.16 2.82 3.06 5.23 4.25 1.1*10 -1 163.0 300 5.38 97.94 28.2 3.07 5.06 4.3 1.1*10 -1 170.3 30 5.66 69.87 28.2 3.04 4.83 4.6 1.1*10 -1 18Fig.17

Abubaker EdkymishTRITA LWR Degree ProjectFigure 16: Adsorption of aluminum (300 µM) added to HFO (3.0 mM) in the presenceof lead (0.00282 mM). Points are measured data, and the line is modeled data bythe CD-MUSIC surface complexation model.Figure 17: Adsorption of aluminum (300 µM) added to HFO (3.0 mM) in the presenceof lead (0.0282 mM) binary-sorbate system. Points are measured data, and theline is modeled data by the CD-MUSIC surface complexation model.18

Lead binding onto iron oxides (Ferrihydrite) as influenced by aluminium and phosphorusFigure 18: Adsorption of aluminum (30 µM) added to HFO (0.3 mM) in the presenceof lead (0.0282 mM) binary-sorbate system. Points are measured data, and the line ismodeled data by the CD-MUSIC surface complexation model.Figure 19: Comparison between Al +3 single adsorption at an Al (T)/Fe ratio of 1.0*10 -1(blue rhombus marks), and binary adsorption (red triangle marks) in the presence of2.82 µM Pb +2 .19

Abubaker EdkymishTRITA LWR Degree ProjectFigure 20: Comparison between Al +3 single adsorption at an Al (T)/Fe ratio of 1.0*10 -1(blue rhombus marks), and binary adsorption (red triangle marks) in the presence of28.2 µM Pb +2 and 3.0mM Fe.Figure 21: Comparison between Al +3 single adsorption at an Al (T)/Fe ratio of 1.0*10 -1(blue rhombus marks), and binary adsorption (red triangle marks) in the presence of28.2 µM Pb +2 and 0.3mM Fe.20

Lead binding onto iron oxides (Ferrihydrite) as influenced by aluminium and phosphorusTable 11: Aqueous solution concentrations in PO 4-Pb binary systems:TotalFemMInitialconc.mMPO 4adsorbedMin.%Max.%Pbinitialconc.µMatMin.AdsorbedPO 4pHatMax.AdsorbedPO 4at50%AdsorbedPO 4RatioofPO 4 (T)/Fe3.0 0.6 54.23 99.44 2.82 7.21 3.14 7.5 2.0*10 -1 223.0 0.6 52.94 99.41 28.2 7.1 3.12 7.5 2.0*10 -1 230.3 0.06 42.41 91.51 28.2 7.55 3.46 7.5 2.0*10 -1 24Fig.Figure 22: Adsorption of phosphate (0.6 mM) added to HFO (3.0 mM) in the presenceof lead (0.00282 mM). Points are measured data, and the line is modeled data bythe CD-MUSIC surface complexation model.21

Abubaker EdkymishTRITA LWR Degree ProjectFigure 23: Adsorption of phosphate (0.6 mM) added to HFO (3.0 mM) in the presenceof lead (0.0282 mM). Points are measured data, and the line is modeled data bythe CD-MUSIC surface complexation model.Figure 24: Adsorption of phosphate (0.06 mM) added to HFO (0.3 mM) in the presenceof lead (0.0282 mM). Points are measured data, and the line is modeled data bythe CD-MUSIC surface complexation model.22

Lead binding onto iron oxides (Ferrihydrite) as influenced by aluminium and phosphorusFigure 25: Comparison between PO 4 single adsorption (initial conc. 0.6 mM) at aP (T)/Fe ratio 2.0*10 -1 (blue rhombus marks), and binary adsorption (red trianglemarks) in the presence of 2.82 µM Pb +2 and 3.0mM Fe.Figure 26: Comparison between PO4 single adsorption (initial conc. 0.6 mM) atP (T)/Fe ratio 2.0*10 -1 (blue rhombus marks), and binary adsorption (red trianglemarks) in the presence of 28.2 µM Pb +2 and 3.0mM Fe.23

Abubaker EdkymishTRITA LWR Degree ProjectFigure 27: Comparison between PO 4 single adsorption (initial conc. 0.06 mM) atP (T)/Fe ratio 2.0*10 -1 (blue rhombus marks), and binary adsorption (red trianglemarks) in the presence of 28.2 µM Pb +2 and 0.3 mM Fe.DISCUSSION:Compared with the research that was published in 2003 in the same area(Swedlund et al 2003) there is a remarkable similarity in some results obtained,for instance at Pb (T)/Fe =1.93*10 -3 and at pH ≈ 3.5 the percentof Pb adsorbed was between 40-45% (Fig. 28) and about 100% at pH≈5, compared with similar conditions in this study (Fig. 1) at Pb/Fe=9.4*10 -4 and at pH ≈ 3.5, during which the percent adsorbed Pb was ≈36% and about 100% at pH ≈ 5.The presence of phosphate increased lead (Pb) removal. The effect wasgreatest at low pH, but at higher pH (6 or 7) the presence of phosphatedid not has an impact since lead (Pb) removal was already approaching100% in the absence of phosphate.It was noticeable from the results obtained that the impact of PO 43- onthe adsorption of Pb 2+ was significant. When the model calibrated forthe single sorbate system was used for this binary system, it proved to bequite unable to describe the data obtained. There are at least two possiblereasons for this:1. Lead (Pb) and phosphate might have precipitated to a lead-phosphatemineral phase i.e. pyromorphite, or2. Lead and phosphate form a ternary complex at the oxide surface,which needs to be accounted for in the model.Because all solutions were under-saturated with pyromorphite, accordingto Visual MINTEQ, alternative (2) was assumed to be the most likelyexplanation.24

Lead binding onto iron oxides (Ferrihydrite) as influenced by aluminium and phosphorusFigure 28: Experimental (symbols) and modeled (lines) adsorption for Pb onto ferrihydritein single sorbate system; adsorption edges (Swedlund et al 2003).Consequently, a reaction of the following type (Eq. 5) was included inthe model:2FeOH+PO 4-3+Pb +2 +2H + ↔ Fe 2O 2PO 2Pb + +2H 2O……...……….. (5)-where, PO 4-3 binds directly to two surface groups of the Fe oxide, andwhere Pb 2+ is then bound to the adsorbed phosphate.The modified model was then able to correctly reproduce the data (Fig.10, 11& 12).Concerning the competitions between Pb 2+ and Al 3+ there was no anysignificant interaction between two ions. From the results it may be seenthat the impact of Al 3+ on the adsorption of Pb 2+ was not significant, anobservation which is also in agreement with the model prediction. This isseen more directly in (Fig. 7, 8 & 9) in which the Pb 2+ adsorption iscompared for systems that are equivalent in all respects except for the Alconcentration. In short there is no clear impact or significant influenceof Al 3+ on the adsorption of Pb 2+ , despite the different in Al 3+ concentrationin the media.In the system (Fig. 25) the influence of Pb 2+ on the adsorption of PO 43-onto ferrihydrite was very small. Between pH 3.2 and pH 4.5 there wasno impact, and form pH 4.5 the presence of lead (Pb) made a small decreaseof adsorbed phosphate onto ferrihydrite.In the system (Fig. 26), the effect of lead (Pb) on the adsorption ofphosphate was also small, and it was only discernible above pH 5.5.In the system (Fig. 27) there was a slight impact, and here the presenceof lead (Pb) caused a some increase of phosphate adsorption onto HFO.25

Abubaker EdkymishTRITA LWR Degree ProjectCONCLUSIONCation competition for adsorption sites is usually accepted (Benjamin &Leckie 1981b), although the magnitude of these effects appears to be different;some have documented considerable competition effects (Harter1991, Zachara et al 1992), while others were reported little effect (O'-Conner & Renn 1964).The cation competition hypothesis has been scientifically tested in anumber of literatures (Posselt et al 1968; Gadde & Laitinen 1974; Zasoski& Burau 1988). Significant indications were obtained by Kurbatov concerningadsorption data of cobalt (Co 2+ ) onto Fe(OH) 3 (Kurbato et al1951). Some researchers found that Ca 2+ was more competitive than Al 3+with adsorption of Eu (Europium), and compared the crystal radii ofAl 3+ and Ca 2+ to evaluate their ability to compete with Eu for adsorptionsites (Clark et al 1998). The absence of strong competition effects in ourcase can be at least partly explained by hydrolysis of the adsorbed Al; themodel could explain the low degree of completion when Al 3+ was adsorbedas Al(OH) 2+ (see reaction in Table 4). If this model description iscorrect, the adsorption of Al 3+ only affects the surface charge to a smallextent, which leads to less competition with other cations including Pb 2+ .On the other hand, a strong effect was observed (in this study) of PO 4-3on the adsorption of Pb 2+ onto ferrihydrite. In earlier literature Spositoin 1986 noted an increased adsorption of metal after a strongly adsorbinganion (Sposito 1986), but Guilherme and Anderson found that the effectof PO 4-3 on adsorption of Cu onto oxide-rich soil is quite complex(Guilherme & Anderson 1998). In case study the strong enhancement ofPb adsorption by phosphorus was explained by the formation of a ternarylead-phosphate complex on the ferrihydrite surface. With these resultsin mind it would be of interest to more closely investigate the effectsof phosphate on metal geochemistry on natural sorbents. Becausephosphate can stabilize metals (at least Pb 2+ ) in this way, it is clear thatphosphorus fertilization of contaminated sites can be an efficient soilamendment method.Finally we can summarize the results: Ferrihydrite is a good adsorbent for Pb 2+ . The presence of PO 43- in the media containing HFO plays as a significantrole for increasing the adsorption ratio of Pb 2+ onto HFO. Adsorption of Pb 2+ onto HFO was not significantly influenced byAl 3+ . Pb 2+ did not have any significant influence on the adsorption of Al 3+and PO 43- onto HFO.The presence of ferrihydrite (HFO) leads to a decrease of Pb concentrationsin the environment, and may render Pb less bio-available for organisms.In situ incorporation of chemicals such as PO 43- with ferrihydritewill enhance HFO adsorption of Pb. The PO 43- used for inactivation alsomay increase the fertility of the soil and eliminate any toxicity to plantsand soil organisms. Further, growing a plant cover may physically stabilizethe soil and its contaminants, which, in order, will minimize soil erosionand the transport of the toxic heavy metals as dust. Although treatmentof contaminated soils with Pb by using phosphate has already beenwidely proposed and accepted as a successful method, the efficiency andsustainability of phosphate application needs more optimization due tothe risk of eutrophication.26

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