Adsorption of humic and fulvic acid on copper surfaces

Arturo Reyes, Marıa Victoria Letelier and Gustavo Lagos

ABSTRACT

The presence of natural organic matter (NOM) in drinking water can increase the levels of

copper released from copper pipes to water and inhibit the formation of protective deposits

such as malachite. Since adsorption of NOM on copper pipes surfaces is believed to be one

of mechanisms that explains this phenomenom, the objective of this study was to determine

kinetics and the adsorption equilibrium of main components of NOM, humic acid (HA) and

fulvic acid (FA), onto copper surfaces. The kinetics and equilibrium adsorption of HA and FA

on copper foils were examined using batch experiments at 221C. HA and FA followed

pseudo second-order kinetics adsorption. Rate constants measured were 2.59� 10�1

(mgTOC cm�2 h�1) for HA and 3.13�10�1 (mgTOC cm�2 h�1) for FA. The adsorption behavior

of HA and FA on the copper surface is in accordance with the Langmuir adsorption isotherm.

Langmuir adsorption constants measured were 5.98� 10�2 L mg�1 for HA and 4.78�10�2 L

mg�1 for FA. The copper foils exposed during five months to FA formed malachite deposits,

whereas those exposed to HA did not and just cuprite was found. The results of this study

showed that both HA as well as FA adsorption on copper surfaces is favored and no

significant differences were found in the adsorption parameters calculated for both

compounds. However, the inhibition of the malachite precipitation could be attributed to the

HA adsorption.

Key words 9999 adsorption, copper, fulvic acid, humic acid

INTRODUCTION

The natural organic matter (NOM) contained in drinking

water has significant effects on all aspects of copper pipe

corrosion (Korshin et al. 1996; Edwards & Sprague 2001; Li

et al. 2004). For instance, the NOM increment the copper

concentrations released from the copper pipes to the water,

and inhibit the precipitation of protective corrosion scales on

copper pipe surfaces, such as malachite (Korshin et al. 1996).

The mechanisms proposed to explain these phenomenom,

includes formation of very stable Cu-NOM complexes and

adsorption of NOM on copper pipes surfaces (Davis 1984).

However, these results seem to be insufficient because neither

considered the role of adsorption on inhibition of crystal

growth nor the effect of its main components, humic acid

(HA) and fulvic acid (FA).

The use of HA and FA can be useful to identify the

effects of adsorption of each compounds on inhibition of

malachite precipitation. HA and FA are compounds with

different properties. At the present is unknown which part

of NOM affect the corrosion of copper surfaces. The

hypotheses is that the inhibition of malachite’s precipita-

tion on copper surfaces exposed to drinking water con-

taining NOM, could be due to either adsorption of HA or

FA. The objective of this study was to determine the

kinetics and adsorption isotherms of HA and of FA on

Arturo ReyesMarıa Victoria LetelierGustavo LagosCentro de Minerıa,Escuela de Ingenierıa,Pontificia Universidad Catolica de Chile,Santiago,Chile

Arturo Reyes (corresponding author)Centro de Minerıa,Escuela de Ingenierıa,Pontificia Universidad Catolica de ChileAv. Vicuna Mackenna 4860,Macul,Santiago,ChileTel.: 56-2 6865895Fax: 56-2 -6865805E-mail: areyes@ing.puc.cl

doi: 10.2166/ws.2010.632

915 & IWA Publishing 2010 Water Science & Technology: Water Supply—WSTWS 9999 10.6 9999 2010

copper foils exposed to a very aggressive synthetic drink-

ing water.

Natural organic matter

NOM is a mix of heterogeneous macromolecules which

occur in soils, sediments, surface water, and groundwater

(Stevenson 1994). NOM is mainly composed of HA and FA.

The acidic functional groups of NOM such as carboxylic

and phenolic (Stevenson 1994; Piccolo et al. 2001) can bind

metal ions forming very stable complexes (Town & Powell

1993). HA is a higher molecular weight compound that

is soluble at alkaline pH but precipitates at acidic pH

(Perminova et al. 2003), while FA has a relatively low

molecular weight and is soluble in water at all pH values

(Perminova et al. 2003). FA is slightly more acidic, polar,

contains more acidic functional groups, but has lower

aromaticity than HA (Stevenson 1994).

Adsorption of NOM

The surface of minerals can be modified by adsorption

of NOM (Tipping 1981b; Inskeep & Bloom 1986b; Sun et al.

2008). For instance, the adsorbed NOM on mineral

oxide surfaces can inhibit the growth of crystal oxides

(Inskeep & Bloom 1986b). The NOM can be adsorbed on

mineral surfaces through ligand exchange (Gu et al. 1995; Sun

et al. 2008) and surface complex formation (Avena & Koopal

1999).

Effect of pH on NOM adsorption

It is generally found that the adsorption decreases at

increasing pH (Tombacz et al. 2000; Illes & Tombacz

2004, 2006; Wang et al. 2009). Tombacz et al. (2000) studied

the effect of pH and ionic strength on the interaction of HA

with aluminum oxide. They concluded that aluminol OH

groups on the surface of alumina can be replaced by

complex-forming ligands of humate. Illes y Tombacz

(2004) studied the ionic strength dependence of HA adsorp-

tion on magnetite at pH ~5, 8 and 9. The HA has high

affinity to magnetite surface especially at lower pH. Illes y

Tombacz (2006) investigated the pH-dependent adsorption

of HA on magnetite and its effect on the surface charging

and the aggregation of oxide particles. This study revealed

that the HA can modify the surface charge properties of

magnetite depending on the amount of adsorbed polya-

nions. The adsorption of HA is more strong that FA adsorp-

tion although more dependent pH and ionic strength (Wang

et al. 2009).

MATERIALS AND METHODS

Solution preparation

The HA and FA used in this study were Standard Suwannee

River obtained from the International Humic Substances

Society (St. Paul, MN, USA). Stock solutions were prepared

by dissolving 100 mg of either HA or FA in 1 L of sodium

hydroxide 10�3 M and stored in the dark at 41C. The stock

solutions were filtrated through a 0.22 m polycarbonate

membrane filter and the total organic carbon (TOC) were

determined.

Synthetic water at pH 6.1, alkalinity 60 mg/L as CaCO3,

calcium 3.6 mg/L, and chloride 10 mg/L was used as a

background solution, which was dosed with either HA or

FA solutions. The synthetic water was prepared using

reagent-grade chemicals and high-purity water with resistivity

418 MO cm, and residual TOCo0.030 mg/L. This water

was previously reported as very aggressive water towards

copper pipes surfaces (Letelier et al. 2008).

Adsorption experiments

Kinetics and adsorption isotherms experiments were carried

out using copper foils (0.1 mm thickness, purity 99.7%,

Merck). The copper foils were washed with perchloric acid

(0.1 M)) and high-purity water, dried and weighed before use.

The organic matter concentration was measured as TOC. The

non-linear curve fitting was conducted using Microsoft Excel

Software. The adsorbed amounts were normalised for sample

surface area.

TOC concentrations were analyzed with a Phoenix 8000

Tekmar Dorman TOC analyzer, (Ohio, USA). The pH

and temperature measurements were made using portable

instruments from WTW (Weilheim, Germany). High-purity

water was obtained using the Barnstead EasyPure System

916 A. Reyes et al. 9999 Adsorption of humic and fulvic acid on copper surfaces Water Science & Technology: Water Supply—WSTWS 9999 10.6 9999 2010

(Dubuque, IA, USA). Samples were shaken using a Heidolph

Unimax 1010 orbital shaker (Schwabach, Germany). Before

use, the glass material were soaked in a 10–1 M solution of

nitric acid for at least 24 h and rinsed before use with high-

purity water. The handling and analysis of the water samples

were carried out according to APHA, AWWA, and WEF

(1998), and US Environmental Protection Agency standards

(1991).

It was used a temperature of 22C due to is close to the

average temperature used in ANSI /NSF-61 (2371C) stan-

dard test used in test rig experiment, which are design to carry

out studies about the behavior of copper pipes exposed to

drinking water. This value has not relation with the major

temperature in the region.

Kinetic adsorption

Batch experiment was performed at 2270.51C to determine

the reaction time to reach equilibrium adsorption. Copper

foils (1 cm2) were added into 75 ml erlenmeyer flasks, which

contained 50 ml of synthetic water dosed with either HA or

FA. The flasks were agitated in the dark at 100 rpm on an

orbital shaker for 15–840 min, and collected at fixed intervals

in order during this time. Then, the synthetic waters were

analyzed for TOC, pH and dissolved oxygen concentrations.

Three replicates of each experiment were carried out. In

order to analyze the adsorption kinetics of HA and FA on

copper foils, the Lagergren pseudo first-and Ho and McKay

pseudo second-order kinetic models were used (Lagergren

1898; Ho & McKay 1998). The kinetic adsorption data were fit

with both models using Microsoft Excel. The fit of both

Lagergren pseudo first-and Ho and McKay pseudo second-

order kinetic models was tested with an F-test to determine

which model best described the data.

Lagergren pseudo first- order kinetics model

The Lagergren pseudo first- order equation, describes the

kinetics of the adsorption process as follows:

dqt

dt¼ k1ðqe � qtÞ ð1Þ

where k1 is the rate constant of pseudo first-order adsorption

(h�1), and qe and qt (mgTOC gCu�1) are the amounts of

organic matter adsorbed at equilibrium and time t (h), respec-

tively. After integration by applying the initial conditions

q¼ 0 at t¼ 0 and q¼ qt at t¼ t, the nonlinear form of

Equation (1) becomes

qt ¼ qeð1� e�k1tÞ ð2Þ

The pseudo first-order rate constant k1 and equilibrium

adsorption qe were evaluated from the linearised form of

Equation (2), represented by the equation:

logðqe � qtÞ ¼ logqe �k1t

2:303ð3Þ

Ho and McKay pseudo second-order kinetics model

The Ho and McKay pseudo second-order kinetics equation,

can be expressed as

dqt

dt¼ k2ðqe � qtÞ2 ð4Þ

where k2 (mgTOC gCu�1 h�1) is the rate constant for pseudo

second-order adsorption. An integrated pseudo-second

order rate law can be obtained from Equation (4) for the

boundary conditions t¼ 0 to t¼ t and qt¼ 0 to qt¼ qt, and is

given by:

qt ¼q2

ek2t1þ k2qet

ð5Þ

Equation (5) can be rearranged to obtain a linear form:

tqt¼ 1

k2q2eþ 1

qet ð6Þ

The pseudo second-order rate constant k2 and equili-

brium adsorption qe were calculated from the slope and

intercept of this equation.

Adsorption isotherms

Batch experiments to determine isotherms were performed

by adding copper foils (1 cm2) to a 75 ml erlenmeyer flasks

containing 50 ml of synthetic water dosed with either

917 A. Reyes et al. 9999 Adsorption of humic and fulvic acid on copper surfaces Water Science & Technology: Water Supply—WSTWS 9999 10.6 9999 2010

HA or FA solution. The adsorption experiments were

conducted with eight concentration points (0–28 mg/L of

TOC), each point including the blank solution was run in

triplicate. The flasks were shaken at 100 rpm in an orbital

shaker. Copper foils were equilibrated with the series of

solutions in the dark for 240 h, at 2270.51C. Afterward, the

copper foils inside the test flasks were removed and the

water samples were analyzed for TOC. The adsorbed

amounts of TOC were calculated from the mass balance

equation:

q ¼ ðC0 � CÞVm

ð7Þ

where C0 and C (mg/L) are the initial and equilibrium

concentrations of the organic matter, respectively, V (L)

is the volume of solution used in the adsorption experiment,

and m (grams) is the mass of the copper foil.

Langmuir model

The Langmuir isotherm model is commonly used to model

NOM adsorption on mineral surfaces (Langmuir 1916). This

model is valid for monolayer adsorption without interactions

between the adsorbed molecules where the adsorption of

each molecule on the surface has equal adsorption energy.

The Langmuir isotherm is represented by the equation

q ¼ qmaxKC1þKC

ð8Þ

where C is the equilibrium organic matter concentration (mg/

L), q is the amount of organic matter adsorbed (mgTOC

gCu�1), qmax is q for a complete monolayer (mgTOC gCu�1),

K is adsorption equilibrium constant (L mg TOC�1).

The linearised form of Equation (8) can be represented by

the equation:

Cq¼ 1

qmaxKþ C

qmaxð9Þ

A plot of C/q against C gives a straight line with a slope of

1/qmax and an intercept of 1/qmaxK.

The adsorption data were fit with both Langmuir and

Freundlich isotherm models using Microsoft Excel. The fit of

both Langmuir and Freundlich models was tested with an

F-test to determine which model best described the data.

Samples preparation for scanning electron

microscopy (SEM)

Copper foils samples (1 cm2) were fixed on metal supports,

sputter-coated with gold (10 nm) (MED 010, Balzers Union

Limited, Liechtenstein), and observed with a LEO 1420VP

SEM with variable pressure technology (Tokyo, Japan). The

presence of pitting was determined by SEM examination of

copper sections after removing the corrosion products, by

pickling in 10% (w/w) citric acid.

X-ray diffraction (XRD)

Sections of copper (1 cm2) inner surfaces were analysed by

XRD. The analysis was carried out in a Siemens D-5000

diffractometer (Karlsruhe, Germany).

RESULTS AND DISCUSSION

FA and HA kinetics adsorption

Figures 1 and 2 presents the experimental kinetic adsorption

data obtained for HA and FA respectively, which were fitted

to the pseudo first- and second-order kinetic model. The

kinetic parameters determined from model simulations are

listed in Table 1. The HA and FA adsorption data could be

well described by the Ho and McKay pseudo second-order

kinetics equation. Also, the values of qe from the pseudo-first

and second order kinetics are in agreement with experimental

data as seen in Table 1.

It can be seen that HA and FA adsorption approaches

equilibrium after 72 h and 24 h, respectively. Thus, FA

adsorption can achieve equilibrium faster than HA adsorp-

tion, but as the adsorption time progressed HA adsorption

also increases. However, the adsorption rates kinetics con-

stant calculated for both compounds were the same magni-

tude order.

The fast kinetics adsorption constants calculated for HA

and FA are indicative of the great affinity of these compounds

on copper surfaces. The adsorption of HA and FA onto the

918 A. Reyes et al. 9999 Adsorption of humic and fulvic acid on copper surfaces Water Science & Technology: Water Supply—WSTWS 9999 10.6 9999 2010

copper surfaces may be considered to consist of chemisorp-

tion processes.

HA and FA adsorption isotherms

Figures 3 and 4 shows the HA and FA adsorption isotherms

on copper foils surfaces. The adsorption isotherms showed

good fits to the Langmuir equation (solid lines in Figures 3

and 4). The adsorption isotherms for HA and FA showed an

initial slope (at low equilibrium TOC), reaching an equili-

brium plateau as TOC concentration increased. Table 2

shows the Langmuir parameters (qmax and K) for these

adsorption isotherms. The correlation coefficient (R2)

describing the goodness of fit to the linearised Langmuir

model is also indicated in this table. The fit to the Langmuir

model would indicate that adsorption sites are homoge-

neously distributed on the surface of the solid phase.

As it mentioned Zhang & Bai (2003), the high negative

charge of humic substances at pH41.6, indicate that the

adsorption isotherms could be dominated by a monolayer

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

time (h)

HA

ads

orbe

d (m

gTO

C/c

m2 )

Experimental

First order

Second order

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000

Figure 1 9999 Kinetic adsorption of HA on copper surfaces.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

time (h)

FA a

dsor

bed

(mgT

OC

/cm

2 )

Experimental

First order

Second order

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000

Figure 2 9999 Kinetic adsorption of FA on copper surfaces.

919 A. Reyes et al. 9999 Adsorption of humic and fulvic acid on copper surfaces Water Science & Technology: Water Supply—WSTWS 9999 10.6 9999 2010

adsorption process, due to the large electrostatic repulsion

between the adsorbed molecules and the molecules to be

adsorbed.

Effect of exposure time on pH of waters

The effect of exposure time on pH of waters is shown in

Figure 5. The pH of water dosed with NOM increased along

exposure period. The pH increment could be attributed to the

OH ions released to the solution during the NOM adsorption.

The OH ions adsorbed on copper surfaces could be exchange

by functional groups of HA and FA. An additional source of

OH ions is the electrochemical reduction of dissolved oxygen

that causes copper corrosion.

It is known that the difference in size and charging

behavior of FA and HA may lead to different electrostatic

interactions with the oxide surface and therefore to a different

proton co-adsorption and pH dependency of the adsorption.

For instance, it has been suggested that the structure of HA

can undergo various changes from rather linear to coiled or

more spherical configurations (or vice versa) due to the

change of pH, ionic strength, and HA concentration in the

solution (Zhang & Bai 2003; Wen et al. 2007).

Surface analysis

The SEM micrographs of copper foils taken after 20 h and

five months of exposure are shown in Figure 6. The copper

Table 1 9999 Parameters of two kinetic models for HA and FA adsorption on copper foils

Pseudo first-order model Pseudo second-order model

Adsorbate k1 (h�1) qe (mg TOC cm�2) R2 k2 (mg TOC cm�2 h�1) qe (mg TOCcm�2) R2

HA 6.70� 10�2 0.410 0.978 2.59� 10�1 0.440 0.999

FA 7.50� 10�2 0.320 0.684 3.13� 10�1 0.350 0.998

0.00

0.04

0.08

0.12

0.16

0.20

0.24

HA after 24 h (mg TOC/L)

ads

orbe

d am

ount

(m

gTO

C/c

m2 )

ExperimentalModel

22 ºC

0 3 6 9 12 15 18 21 24 27 30

Figure 3 9999 Langmuir Adsorption isotherm of HA on copper surfaces.

0.00

0.04

0.08

0.12

0.16

0.20

FA after 24 h (mg TOC/L)

Experimental

Model

22 ºC

ads

orbe

d am

ount

(m

gTO

C/c

m2 )

0 3 6 9 12 15 18 21 24 27 30

Figure 4 9999 Langmuir Adsorption isotherm of FA on copper surfaces.

Table 2 9999 Langmuir parameters for HA and FA adsorbed on copper foils (221C)

K (Lmg�1) qmax (mg TOC cm�2) R2

HA 5.98� 10�2 0.232 0.999

FA 4.78� 10�2 0.160 0.999

5.00

5.50

6.00

6.50

7.00

7.50

8.00

8.50

9.00

time (h)

pH

HAFA

020

040

060

080

010

0012

0014

0016

0018

0020

0022

0024

0026

0028

0030

00

Figure 5 9999 Effect of exposure time on pH.

920 A. Reyes et al. 9999 Adsorption of humic and fulvic acid on copper surfaces Water Science & Technology: Water Supply—WSTWS 9999 10.6 9999 2010

foil exposed to organic-free water during 20 h did not show

malachite deposits (Figure 6a). However, the typical crystal

malachite structures (Figure 6b) were observed after five

months, which was confirmed by X- ray diffraction. In the

case of copper foil exposed to water dosed with FA, not

malachite deposits were observed after 20 h (Figure 6e).

Interestingly, the copper foil exposed to FA during 5 months

contained malachite deposits (Figure 6f), whereas that

those exposed to HA just contained cuprite deposits (Figure

6c and d).

These results shows that in comparison with FA, the HA

totally inhibited malachite precipitation. This result suggest

that the inhibitory effect of malachite precipitation could be

due to the HA presence. The inhibition mechanism could be

the adsorption on copper surfaces.

The total inhibition of malachite’s precipitation in the

presence of HA regarding to FA can be due to the different

properties of these compounds. Possibly the HA could favor

the formation of amorphous compounds with poor protective

properties against corrosion. A consequence of this phenom-

enom can be observed in the copper pipes surfaces exposed

to drinking water that contain organic matter. These surfaces

are not covered by protective corrosion scales, which cause

the release of copper to the water. Under these conditions, the

drinking water could exceed the limits of copper established

by World Health Organization to human consumption

(2 mg/L of copper).

CONCLUSIONS

The results of this study showed that both HA as well as FA

adsorption on copper surfaces is favored and not significant

differences were found in the adsorption parameters

5 months20h

Organic-free water

HA

FA

(a) (b)

(c) (d)

(e) (f)

Figure 6 9999 SEM micrograph of copper foils exposed to water without organic matter, HA and FA after 4 months equilibrium period. Magnification 1000� .

921 A. Reyes et al. 9999 Adsorption of humic and fulvic acid on copper surfaces Water Science & Technology: Water Supply—WSTWS 9999 10.6 9999 2010

calculated for both compounds. The adsorption behavior

of HA and FA on the copper surface is in accordance with

the Langmuir adsorption isotherm. Langmuir adsorption

constants measured were 5.98� 10�2 Lmg�1 for HA and

4.78� 10�2 Lmg�1 for FA. HA and FA followed pseudo

second-order kinetics. Rate constants measured were

2.59� 10�1 (mgTOC cm�2 h�1) for HA and 3.13� 10�1

(mgTOC cm�2 h�1) for FA. The inhibition of the malachite

precipitation could be attributed to the HA adsorption.

ACKNOWLEDGEMENTS

This research was supported by a grant provided by the

International Copper Association, and the Comision Chilena

del Cobre (Cochilco).

REFERENCES

APHA, AWWA, and WEF. 1998 Standard Methods for the Examinationof Water and Wastewater, 20th ed.. American Public HealthAssociation/American Water Works Association/Water Environ-ment Federation, Washington, DC.

Avena, M. J. & Koopal, L. K. 1999 Kinetics of humic acids adsorption atsolid-water interfaces. Environmental Science and Technology 33,567–573.

Davis, J. A. 1984 Complexation of trace metals by adsorbed naturalorganic matter. Geochimica et Cosmochimica Acta 48(4),679–691.

Edwards, M. & Sprague, N. 2001 Organic matter and copper corrosionby-products release: A mechanistic study. Corrosion Science 43(1),1–18.

Gu, B., Schmitt, J., Chen, Z., Liang, L. & McCarthy, J. F. 1995 Adsorp-tion and desorption of different organic matter fractions on ironoxide. Geochimica et Cosmochimica Acta. 59, 219–229.

Ho, Y. S. & McKay, G. A. 1998 Comparison of chemisorption kineticmodels applied to pollutant removal on various sorbents. Trans.Inst. Chem. Eng. 76B, 332–340.

Illes, E. & Tombacz, E. 2004 The role of variable surface charge andsurface complexation in the adsorption of humic acid on magne-tite. Colloids and Surfaces A: Physicochemical and EngineeringAspects 230, 99–109.

Illes, E. & Tombacz, E. 2006 The effect of humic acid adsorption onpH-dependent surface charging and aggregation of magnetitenanoparticles. Journal of Colloid and Interface Science 295,115–123.

Inskeep, W. P. & Bloom, P. R. 1986b Kinetics of calcite precipitation inthe presence of water-soluble organic ligands. Soil Science Societyof America Journal 50, 1167–1172.

Korshin, G. V., Ferguson, J. F. & Perry, S. A. L. 1996 Influence of NOMon copper corrosion. Journal of American Water Works Associa-tion 88(7), 36–47.

Lagergren, S. 1898 Zur theorie der sogenannten adsorption gelosterstoffe. 591. Kungliga Svenska Vetenskapsakademiens, Handlingar.24, 1–39.

Langmuir, I. 1916 The Constitution and Fundamental Properties of Solidsand Liquids. Journal American Chemical Society 38, 2221–2295.

Letelier, V., Lagos, G. & Reyes, A. 2008 Chemical characterization ofblue stains in domestic fixtures in contact with drinking water.Environmental Monitoring and Assessment 139, 307.

Li, S., Ni, L., Sun, C. & Wang, L. 2004 Influence of organic matter onorthophosphate corrosion inhibitionfor copper pipe in soft water.Corrosion Science 46, 137–145.

Perminova, I. V., Frimmel, F. H., Kudryavtsev, A. V., Kulikova, N. A.,Abbt – Braun, G., Hesse, S. & Petrosyan, V. 2003 MolecularWeight Characteristics of Humic Substances from Different Envir-onments As Determined by Size Exclusion Chromatography andTheir Statistical Evaluation. Environmental Science and Techno-logy 37, 2477–2485.

Piccolo, A., Conte, P., Cozzolino, A. & Spaccini, R. 2001 Compositionand Structure. In: Humic substances and chemical contaminants,Section: 1, Chapter 4 Clapp, C. E., Hayes, M. H. B., Senesi, N.,Bloom, P., Jardine, P. M. (eds.), Soil Science Society of America,Inc, Madison, Wis.

Stevenson, F. J. 1994 Humus Chemistry, Genesis, Composition, Reac-tions, 2th ed. John Willey and Sons, New York.

Sun, X.- F., Wang, S.- G., Liu, X.- W., Gong, W.- X., Bao, N. & Ma, Y.2008 The effects of pH and ionic strength on fulvic acid uptake bychitosan hydrogel beads. Colloids and Surfaces A: Physicochem-ical and Engineering Aspects 324, 28–34.

Town, R. M. & Powell, H. K. J. 1993 Ion-selective electrode potentio-metric studies on the complexation of copper (II) by soil-derivedhumic and fulvic acids. Analytica Chimica Acta. 279, 221–233.

Tipping, E. 1981b The adsorption of aquatic humic substances by ironoxides. Geochimica et Cosmochimica Acta. 45, 191–199.

Tombacz, E., Dobos, A., Szekeres,M., Narres, H. D., Klumpp, E. &Dekany, I. 2000 Effect of pH and ionic strength on the interactionof humic acid on the interaction of humic acid with aluminiumoxide. Colloid Polymer Science 278, 337–345.

US Environmental Protection Agency 1991 Methods for the Determina-tion of Metal in Environmental Samples, Environmental SystemsLaboratory, Office of Research and Development, The documentused was revised in 1994, revision 5.4, Cincinnati.

Wang, S., Hu, J., Li, J. & Dong, Y. 2009 Influence of pH, soil humic/fulvic acid, ionic strength, foreign ions and addition sequences onadsorption of Pb(II) onto GMZ bentonite. Journal of HazardousMaterials 167, 44–51.

Wen, B., Zhang, J. J., Zhang, S. Z., Shan, X. Q., Khan, S. U. & Xing, B. S.2007 Phenanthrene sorption to soil humic acid and different huminfractions. Environmental Science & Technology 41, 3165–3171.

Zhang, X. & Bai, R. 2003 Mechanisms and kinetics of humic acidadsorption onto chitosan-coated granules. Journal of ColloidsInterface Science 264, 30–38.

922 A. Reyes et al. 9999 Adsorption of humic and fulvic acid on copper surfaces Water Science & Technology: Water Supply—WSTWS 9999 10.6 9999 2010