A

IFibacMpM©

K

1

(iappAhKn

womcnr

1d

Biochemical Engineering Journal 39 (2008) 478–485

Study on biosorption of humic acid by activated sludge

Hua-Jun Feng a, Li-Fang Hu a, Qaisar Mahmood b, Yan Long a, Dong-Sheng Shen a,∗a College of Environmental and Resource Sciences, Zhejiang University, 268 Kaixuan Road, Hangzhou 310029, PR China

b Department of Botany, Federal Government Post Graduate College, Sector H-8 Islamabad, Pakistan

Received 23 November 2006; received in revised form 4 September 2007; accepted 11 November 2007

bstract

The biosorption performance of humic acid (HA) by activated sludge and the effects of inorganic metal ions as biosorption aid were investigated.t was confirmed that biosorption instead of biodegradation was responsible for HA removal. HA biosorption by activated sludge followed thereundlich isotherm equation; however, biosorption of HA corresponded to the biosorption by a mixture of adsorbates. The removal efficiency

mproved when initial HA concentration was increased. While biosorption of HA can be suitably applied to treat high concentration HA wastewaterecause the HA concentration of blank sample was approximate to 1 mg/L. The total removal efficiency for HA improved when the quantity ofdsorbent (activated sludge) was increased under constant contact time. The lower concentration of the adsorbent could realize the higher adsorptiveapacity per gram activated sludge with a longer contact time, but the removal rate of pollutants was more important consideration in practice.

oreover, biosorption of HA was favored by addition of five metallic salts. The ionic strength increasing, cationic bridge, destabilization and

recipitation are the main mechanisms to advance HA removal efficiency in solution after adding the salt (Na+), alkaline earth metallic (Ca2+,g2+) and hydrolyzing metallic (Al3+, Fe3+) ions, respectively.2007 Elsevier B.V. All rights reserved.

try; H

bthecobNucau

br

eywords: Activated sludge; Biosorption; Biodegradation; Biophysical chemis

. Introduction

The humic acid (HA) fraction of natural organic matterNOM) is considered problematically in drinking water becauset can react readily with chlorine to form carcinogen compoundsnd it can form complex with heavy metals and hydrophobicolychlorinated organics [1], influencing their fate and trans-ort [2]. It possibly has negative impact on human health.ctually, there exist two reports on endemic diseases that arearmful to those who used to drink well water near peat bogs:aschin–Beck disease, a chronic osteoarthritic disorder withecrosis of condrocytes prevailing in China [3].

Several physico-chemical wastewater treatment techniquesere developed and successfully applied to reduce NOM contentf water such as radiation [4], flocculation with synthetic poly-ers [5], membrane filtration [6], and adsorption with activated

arbon [7]. However, the application of such treatment tech-iques either needs enormous cost and presents an unsatisfactoryemoval capacity or causes secondary pollution [1,8].

∗ Corresponding author. Tel.: +86 571 86971156; fax: +86 571 86945370.E-mail address: shends@zju.edu.cn (D.-S. Shen).

f

feteo

369-703X/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.bej.2007.11.004

umic acid; Wastewater treatment

In recent years, some studies have been carried out oniosorption, a biophysico-chemical process to remove pollu-ants from solution by attachment on microbial surfaces. Manyeavy metals and organic contaminants have been successfullyliminated from solution through biosorption under differentonditions [9,10]. Though some investigations on biosorptionf HA were carried out, most of them used dried [11] or immo-ilized biological adsorbents [8] to investigate the removal ofOM from wastewater. Few studies reported biosorption of HAsing activated sludge, especially about some factors (e.g. initialoncentration of HA in the solution, biological adsorbent dosagend inorganic metallic ions) that may influence HA biosorptionsing activated sludge.

Another benefit of employing activated sludge is that it cane used for the production of fertilizers after treatment of HAich wastewater because the application of humic substances asertilizers is very effective in crop production [3].

The objective of the present study was to investigate the per-ormance of HA biosorption using activated sludge in batch

xperiments and the effects of inorganic metal ions on biosorp-ion. To achieve the objectives, the biosorption rate and removalfficiency of HA were examined using different concentrationsf HA, biomass and inorganic metallic ions.

ginee

2

2

faFCwsbBa

2

aidsrwa

2

2

m(oews2ctwtwabp

2

cpmHmr0to

a(

2

2

tdid1

1dcrwm1absfct

C

wtmao

C

wCca

q

wVs

wi

L

w

H.-J. Feng et al. / Biochemical En

. Materials and methods

.1. HA stock solution

Humic acid (sodium salt, technical grade) was purchasedrom Aldrich and used without further purification. For char-cterization, it was converted to the acid form HA followingukushima et al. [12]. The results of the elemental analysis were:= 49.65%, H = 4.49%, N = 0.96%, S = 0.33%, and ash contentas 2.46%. Stock solution of this salt was prepared daily by dis-

olving an aliquot of HA into deionized water using ultrasonicath. The pH was adjusted at 7.2 ± 0.2 by 0.1 mol/L NaOH.atch adsorption experiments were conducted at (28 ± 1 ◦C) inroom containing thermostat device.

.2. Activated sludge collection

The activated sludge with pH of 7.2 was collected from theeration tank in Hangzhou Sibao Wastewater Treatment Plantn Zhejiang Province, China. After the supernatant fluid wasecanted off, the total suspend solids (TSS) and volatile suspendolids (VSS) of sludge were 9.89 ± 1.21 and 5.81 ± 0.01 g/L,espectively. After collection, the sludge was stored at 4 ◦Cithout any pretreatment in order to preserve the original char-

cteristics until the application.

.3. Analytical methods

.3.1. Resin cleaningResin cleaning was performed according to the previous

ethod [13]. In order to remove dissolved organic carbonDOC), Amberlite XAD-2 resin (40 meshed, which wasbtained in cubic-foot quantities from Rohm and Hass) wasxtracted in a 1 L beaker containing 0.1 mol/L NaOH. Finesere decanted off after each daily rinsing of NaOH for 5 succes-

ive days. Then, the resin was Soxhlet-extracted sequentially for4 h with methanol, and stored in methanol before use. Prior toolumn packing, methanol was rinsed with distilled water con-aining resin in a large beaker using slurry technique. The resinas packed as methanol–water slurry and then rinsed with dis-

illed water until it became free of methanol. The packed columnas now rinsed thrice, through alternate use of 0.1 mol/L NaOH

nd 0.1 mol/L HCl; this removed impurities that may otherwisee incorporated into the sample. It was paramount that rinsingreceded the sample application.

.3.2. HA determination [14]Glenco glass columns (ϕ10 mm × 200 mm) were used for

olumn chromatography, and samples were collected using aeristaltic pump. Firstly, samples were filtered through 0.45 �membrane and their pH was adjusted at 2.0 with 1 mol/LCl; then the 0.25 L of samples (or 0.01 L, which was deter-ined by sample concentration) were passed through XAD-2

esin column at a flow rate of 4 mL/min; thirdly, 50 mL of.1 mol/L HCl were passed through the elute column in ordero remove cations. Finally, the elute column containing 50 mLf 0.1 mol/L NaOH at a flow rate of 4 mL/min was collected

Tctc

ring Journal 39 (2008) 478–485 479

nd was measured at 254 nm by Ultraviolet spectrophotometerUV-9100).

.4. Test procedures

.4.1. Effect of various initial HA concentrationsTo explore the underlying mechanism of HA removal

hrough biodegradation and biosorption, two groups of test wereesigned, i.e. one employed activated sludge and the other usednactivated sludge. The inactivated sludge was produced byeactivation of activated sludge with penicillin (200 U/L) for2 h before use.

Hundred milliliters of sludge (activated or inactivated),50 mL deionized water and 250 mL HA stock solutions ofifferent concentrations were added in a 1 L jar and then theontents were agitated on a temperature-controlled shaker at theate of 150 rpm. The HA concentrations in the final solutionsere 5, 10, 20, 40 and 60 mg/L, respectively. Triplicate experi-ents were conducted at nine different contact times (1, 2, 4, 8,

6, 32, 48, 72 and 96 h). Ultraviolet adsorption was measuredt 254 nm for each sample. Blanks with the same amount ofiomass (without HA) were run in triplicate along the sorptionamples to correct the final HA concentration in each sampleor DOC. The final dissolved phase concentration of HA wasalculated by subtracting the average of the blank samples fromhe average of the HA samples

= Csample − Cblank (1)

here C is the HA concentration in the mixture at various contactimes (mg/L); Csample (mg/L) is the average HA concentration

easured in biosorption samples and Cblank (mg/L) is the aver-ge HA concentration measured for blank samples. The amountf HA absorbed was estimated as follows

sorbed = C0 − C (2)

here C0 is the initial HA concentration in sorption samples andis the concentration calculated using Eq. (1). The adsorptive

apacity of HA per gram sludge in the mixture was calculateds follows

= Csorbed × V

TSS(3)

here Csorbed is the concentration calculated using Eq. (2), andis the mixture volume (0.5 L) and TSS stands for the total

uspend solids (9.89 g/L).The Freundlich adsorption isotherm equation is the most

idely used empirical mathematical description of adsorptionn an aqueous medium. The Freundlich equation is expressed as

og(q) = log K + 1

n log Ce(4)

here q is the adsorbed amount of HA per gram sludge (mg/g-

SS), and is calculated using Eq. (3). Ce is the solute equilibriumoncentration (mg/L); and K and n are constant characteristic ofhe system that can be considered as an indicator of adsorptiveapacity and intensity, respectively [9].

480 H.-J. Feng et al. / Biochemical Engineering Journal 39 (2008) 478–485

F ation.1 udge a

2

saostot1toewassco

2

la7(toa1cUp(co

3

3

ibctrtlr

The contact time to reach pseudo-equilibrium extended withthe increasing initial HA concentrations (Fig. 2). It took 48 hfor HA removal efficiency to reach relatively constant at ini-tial concentration of 5–40 mg/L, whereas 96 h for HA initial

ig. 1. HA concentration profile in the solution at different initial HA concentr0 mg/L (�), 20 mg/L (�), 40 mg/L (♦), 60 mg/L (�). (A) Activated aerobic sl

.4.2. Effect of different activated sludge concentrationsIn order to reveal the effects of different adsorbent (activated

ludge) concentrations on HA adsorption, 50, 100 and 150 mLctivated sludge volumes were mixed with 200, 150 and 100 mLf deionized water, respectively, in a 1 L jar, then 250 mL of HAtock solution (80 mg/L) was supplemented to obtain the mix-ures of same total volume but having different concentrationsf sludge. The HA concentration was 40 mg/L in final solu-ions and the final sludge concentrations were 0.989 g-TSS/L,.978 g-TSS/L and 2.967 g-TSS/L, respectively. The experimen-al jars were agitated on a temperature-controlled shaker at ratef 150 rpm. Triplicate experiments were conducted at five differ-nt contact times i.e. 1, 8, 24, 48 and 96 h. Ultraviolet adsorptionas measured at 254 nm for each sample. Blanks with the same

mount of biomass (without HA) were run in triplicate along theorption samples to correct the final HA concentration in eachample for desorbed organic matter. The final dissolved phaseoncentration of HA was calculated by subtracting the averagef the blank samples to the average of the HA samples.

.4.3. Effect of different inorganic metallic ionsHundred milliliters of activated sludge and 150 mL of metal-

ic salts solution (NaCl, MgCl2, CaCl2, FeCl3 and AlCl3) weredded in a jar of 1 L capacity and the pH was adjusted at.2 ± 0.2 by 1 mol/L NaOH, then 250 mL of HA stock solution80 mg/L) was supplemented to the jar. The jars containing con-ents were agitated on a temperature-controlled shaker at the ratef 150 rpm. The HA concentration in final solutions was 40 mg/Lnd the final concentrations of various metallic salts were 10−1,0−2 and 10−3 mol/L, respectively. Triplicate experiments wereonducted at five different contact times (1, 4, 8, 32 and 48 h).

ltraviolet adsorption was measured at 254 nm for each sam-le. Blanks with the same amount of biomass and metallic saltswithout HA) were run in triplicate along the sorption samples toorrect the final HA concentration in each sample for desorbedrganic matter.

FA

The initial HA concentrations in the final solutions were 0 ( ), 5 mg/L (©),nd (B) inactivated aerobic sludge.

. Results and discussion

.1. Removal mechanism and kinetics

The kinetics of HA biosorption onto biomass (activated andnactivated sludge) were shown in Fig. 1. Paired t-tests (α = 0.05)ased on the average difference between various initial HA con-entrations of activated and inactivated sludge were calculatedo determine whether the difference was significant or not. Theesults showed that the difference between activated and inac-ivated sludge was non-significant (P > 0.05). Therefore, it isogical to deduce that biosorption instead of biodegradation wasesponsible for HA removal.

ig. 2. Adsorption of HA per gram sludge at different initial HA concentration.ctivated aerobic sludge (�) and inactivated aerobic sludge (©).

ginee

clmaspfltb(TtHbs

3

saumhtLuotas

rbmvvi

Fv

3

3

stcctlasottpc4tcvewtdtwea

3

swfor the initial HA concentration of 5 mg/L. It improved fur-

H.-J. Feng et al. / Biochemical En

oncentration of 60 mg/L. Based on the previous study [9], theive activated sludge sorbent kept releasing continuously organic

atter into solution and part of the HA may also be biodegrad-ble, although the kinetics of biodegradation may be muchlower than for simple substrates [15]. So, even after reachingseudo-equilibrium, the HA concentration in the solution mayuctuate to some extent. To avoid or lessen such interference,

he HA concentrations at pseudo-equilibrium were calculatedy averaging the HA concentrations at various contact times48, 72 and 96 h) for 5–40 mg/L initial HA concentrations.he HA concentration for the contact time of 96 h was equal

o the concentration at pseudo-equilibrium for 60 mg/L initialA concentration because the difference was non-significantetween the concentration at 96 and 120 h (P > 0.05) (data nothown).

.2. Biosorption isotherms

Generally, studies on adsorption aimed at further under-tanding on mechanisms and adsorptive capacity of adsorbentst constant operating conditions, so the two most commonlysed adsorption isotherms for biosorption studies, the Lang-uir and Freundlich adsorption isotherms, were investigated

ere. Although previous studies demonstrated that the biosorp-ion of heavy metallic ions by activated sludge followed theangmuir mode [8], the present study showed that the Fre-ndlich isotherm equation (Fig. 3) fitted better than Langmuirne (data not shown). It was necessary to point out that biosorp-ion of HA corresponded to the biosorption of a mixture ofdsorbates because of the heterogeneous character of theseubstances.

Different types of sludge can cause distinct results for HAemoval due to diverse biomass characteristics, such as micro-ial species involved, cellular fraction, culture age and cultureedium used that may influence the adsorptive capacity acti-

ated sludge for HA significantly. It was also reported that the Kalue for Freundlich isotherm was 0.3–0.4 at pH 8.0 [9], whereasn present study the K value was 1.9–2.4 at pH 7.2.

ig. 3. Freundich isotherms of the adsorption of HA by activated sludge. Acti-ated aerobic sludge (�) and inactivated aerobic sludge (©).

tat

Fa

ring Journal 39 (2008) 478–485 481

.3. The factors affecting HA biosorption

.3.1. The effect of pHThe pH is an important factor influencing HA removal. Many

tudies reported that the ability of HA removal increased withhe decreasing pH either for dried sludge [16,17] or for activatedarbon [18]. Therefore, it was hypothesized that there existedommon physico-chemical principles underlying these biosorp-ion processes. During practical operation, the pH may presentess variability owing to the natural buffering capacity of waternd the growth conditions of microorganism involved. So, theolution pH in the study was set according to the original pHf activated sludge without any adjustment (pH 7.2) except foresting inorganic ions. Adding different inorganic ions into solu-ion, especially hydrolyzing metallic cations, may result to theH decline due to hydrolysis reactions. The optimal pH of pre-ipitation for Al3+ and Fe3+ was in the range of 5.6–7.1 and.4–6.7, respectively [19]. But in order to study the effects ofhe inorganic metallic ions on biosorption, identical operationalonditions (pH and temperature) were required. In fact, the acti-ated sludge system displayed natural buffering capacity to somextent, so it was unnecessary to adjust the pH of solutions excepthen Al3+ and Fe3+ were added. After adding Al3+ and Fe3+ at

he concentration of more than 10−2 mol/L resulted in the pHecrease drastically. Therefore, in order to keep identical condi-ions, the pH was adjusted to 7.2 ± 0.2 by 1 mol/L NaOH, whichas a decisive factor in HA solubility. Therefore, the removal

ffect can be further improved provided that the pH is optimalfter adding Al3+ and Fe3+.

.3.2. Effect of initial HA concentrationsAs HA biosorption reached pseudo-equilibrium, the relation-

hip between initial HA concentration and removal efficiencyas shown in Fig. 4. The removal efficiency improved to 78.7%

her to 96.6% at the initial HA concentration of 60 mg/L. Inddition, the adsorptive capacity of HA per gram sludge andhe initial HA concentration fitted a linear relationship at the

ig. 4. Relationship between initial HA concentration and removal efficiency,dsorptive capacity per gram sludge in 96 h.

482 H.-J. Feng et al. / Biochemical Enginee

Fig. 5. Adsorption of HA by different sludge concentrations. The concentrationof activated aerobic sludge in the final solutions were 0.989 g-TSS/L (HA con-centration in solution (�), HA concentration of blank (�), HA removal efficiency(bi

ptcbcabH

3

acpsd

Fcw

TcaAacta(tpisata

3

3

itoWtpcrTA

cw

©)); 1.978 g-TSS/L (HA concentration in solution (♦), HA concentration oflank (♦), HA removal efficiency (�)) and 2.967 g-TSS/L (HA concentrationn solution (�), HA concentration of blank ( ), HA removal efficiency (�)).

seudo-equilibrium (Fig. 4). The curve indicated that the adsorp-ive capacity of HA increased with the increasing initial HAoncentration. Therefore, this process of HA biosorption cane suitably applied to treat the wastewater with high HA con-entration because the HA concentration of blank simple waspproximately 1 mg/L. Some other additional adsorbent shoulde added or post-treatment process may be adopted to removeA completely.

.3.3. Effect of different activated sludge concentrationsActivated sludge volumes of 50, 100 and 150 ml were

pplied during the tests to reveal the effect of different sludge

oncentrations on biosorption of HA. Upon approaching theseudo-equilibrium, the largest HA adsorptive capacity per gramludge was presented for the least adsorbent (0.989 g-TSS/L),espite the least amount of the adsorbed HA (Figs. 5 and 6).

ig. 6. Adsorption of HA per gram sludge at different activated aerobic sludgeoncentration. The activated aerobic sludge concentrations in the final solutionsere 0.989 g-TSS/L (�), 1.978 g-TSS/L (©), and 2.967 g-TSS/L (�).

Trliwtmteottttt

3

w

3tN

ring Journal 39 (2008) 478–485

he adsorption isotherm is a useful indication of the adsorptiveapacity of an adsorbent for an adsorbate, which reveals whethern adsorbent reaches its maximum adsorptive capacity or not.n equally important parameter in an engineering system, such

s water treatment process, is the adsorption rate. The shorterontact time, the better efficiency realized. Hence, the adsorp-ion rate can be more important than the ultimate amount ofdsorption to some extent [7]. The concentration of adsorbateHA) and adsorbent (activated sludge) in solution may affecthe rate of adsorption, and the contact time upon reaching theseudo-equilibrium of biosorption that decreased with increas-ng quantity of adsorbent since adsorbent determined the totalurface areas available for solute adsorption. Therefore, as fars engineering application is concerned, the shorter contact timeo reach pseudo-equilibrium is preferred, the higher ratio of thedsorbent and adsorbate (HA) may be recommended.

.4. Effect of different inorganic cations

.4.1. PerformanceThe quantity of HA biosorption increased in the presence of

norganic cations (Table 1). Similar effects have been reported inhe previous studies during the biosorption of HA in the presencef metallic salts by dried sludge and activated carbon [9,11].eber and co-workers found that for concentrations as at typical

ap water levels, metallic cations such as Ca2+, Mg2+, and Fe3+,roduced pronounced effects on the equilibrium capacities of thearbon for HA [7]. The ionic strength has positive effect on HAemoval after adding Ca2+ and Na+ into the feed solution [9,20].herefore, five inorganic salts (NaCl, MgCl2, CaCl2, FeCl3 andlCl3) were chosen to study their effects on the HA biosorption.The interaction between the metallic ions and the HA can

ause a superiority in the overall HA removal process comparedith biosorption alone after adding metallic cations (Table 1).he HA adsorption total removal efficiency or contact time of

eaching pseudo-equilibrium was favored when inorganic metal-ic ions were added. However, the performance of these five saltsn HA removal was differential. For Na+, the removal efficiencyas favored with the increasing salt concentration and contact

ime. For alkali metallic ions like Ca2+ and Mg2+, the most opti-al concentration for removing HA was 10−1 mol/L for three

ested concentration and the contact time to reach the pseudo-quilibrium was less than 1 h. On the whole, the removal effectf Ca2+ was better than that of Mg2+. The most optimal concen-ration of Al3+ and Fe3+ for removing HA was 10−2 mol/L, andhe removal efficiency was nearly 100%. Meanwhile, the adsorp-ion and HA removal was not affected by the contact time. Onhe contrary, when the concentrations of Al3+ and Fe3+ were upo 10−1 mol/L, the HA removal efficiency dropped.

.4.2. MechanismBiosorption of HA was favored by adding five metallic salts

hereas their removal mechanisms were different.

.4.2.1. Ionic strength. Ionic strength had a positive effect onhe adsorptive capacity of activated sludge biomass. Addinga+ increased the ionic strength of the solution, as a result

H.-J. Feng et al. / Biochemical Engineering Journal 39 (2008) 478–485 483

Table 1Effect of biosorption of humic acid in presence of different inorganic salts at concentrations of 10−1, 10−2 and 10−3 mol/L (n = 3, mean ± S.D.)

Metallic cations Concentration (mol/L) HA concentrations in solution (mg/L)

0 ha 1 h 4 h 8 h 32 h 48 h

Na+10−1 40 8.21 ± 0.46 8.77 ± 0.15 5.99 ± 0.50 2.29 ± 0.40 1.43 ± 0.1510−2 40 19.52 ± 0.70 12.95 ± 0.40 10.65 ± 0.85 12.71 ± 0.97 8.10 ± 0.2010−3 40 12.34 ± 1.53 20.33 ± 2.08 17.96 ± 0.74 15.22 ± 0.90 7.35 ± 0.51

Mg2+10−1 40 2.60 ± 0.62 0.33 ± 0.49 1.27 ± 0.61 0.54 ± 0.58 0.63 ± 0.5110−2 40 5.90 ± 0.52 4.03 ± 0.30 4.51 ± 0.30 3.00 ± 0.20 2.32 ± 0.4010−3 40 11.65 ± 0.90 21.09 ± 1.37 16.40 ± 1.01 4.74 ± 1.31 4.67 ± 0.59

Ca2+

10−1 40 0.21 ± 0.10 0.31 ± 0.28 0.13 ± 0.15 0.30 ± 0.20 0.10 ± 0.1010−2 40 2.57 ± 0.62 1.01 ± 0.20 1.54 ± 0.38 0.62 ± 0.26 0.64 ± 0.1710−3 40 8.75 ± 1.50 11.60 ± 0.70 13.78 ± 1.34 3.11 ± 1.35 2.90 ± 0.90

Al3+

10−1 40 0.50 ± 0.43 0.56 ± 0.63 0.57 ± 0.48 0.56 ± 0.57 0.66 ± 0.3910−2 40 0.03 ± 0.06 0.01 ± 0.02 0.09 ± 0.10 0.03 ± 0.06 0.06 ± 0.0510−3 40 13.34 ± 1.60 20.09 ± 1.60 21.30 ± 0.89 5.39 ± 0.60 3.93 ± 0.31

F 3+

10−1 40 0.58 ± 0.20 0.90 ± 0.21 0.76 ± 0.15 0.50 ± 0.10 0.69 ± 0.1910−2 40 0.15 ± 0.18 0.10 ± 0.10 0.03 ± 0.06 0.03 ± 0.06 0.10 ± 0.10

twiblssatiiar

3Csdtaisttahvatcatmom

tbm

3lipwwhettai

Hmstorbiowaaik

e10−3 40 6.42 ± 0.60

a Contact time.

he HA removal efficiency rose. The Derjaugin, Landau, Ver-ey, and Overbeek (DLVO) theory [21,22] can explain the

onic strength effect. At low ionic strength, the activated sludgeiomass and HA have extended electric double layer. The doubleayer becomes compressed at high ionic strength, reducing theurface potential, but keeping the surface charge density con-tant. The compression of the double layer results in a closerpproach of HA and activated sludge biomass, which increasehe adsorptive capacity of biomass. The higher adsorptive capac-ties at higher ionic strength are possible due to decreased totalnteraction forces as the double layer repulsion forces decreaset higher ionic strengths and the Van der Waals attraction forcesemain independent of the solution ionic strength [9].

.4.2.2. Cationic bridge. It is confirmed that the presence ofa2+ and Mg2+ in solution, at concentration levels typical of

urface waters, improved the adsorption of humic substancesramatically. The proposed mechanisms of the enhanced adsorp-ion involve interactions between adsorbate-ion, adsorbent-ionnd adsorbate-ion–adsorbent, although the exact nature of thesenteractions is not very clear [7]. It has been found that in thetructure of HA, carboxyl, phenolic and aliphatic hydroxyl func-ional groups exist as components of the humic acid core, andhose alkaline earth metallic cations are in some way associ-ted with the humic acid molecules. It is also suggested thatumic substances interact with alkaline earth metallic cationsia ion exchange between these cations and the hydrogen ionsssociated with the carboxyl and phenolic hydroxyl groups onhe humic substances [8]. The divalent cations, such as cal-ium, magnesium, etc., may act as a bridge between HA andctivated sludge microorganism because both have anionic func-

ional groups over pH 6 and 8 [23]. Concurrently, alkaline earthetallic cations may neutralize the negative surface charges

f both HA and activated sludge microorganism making eachore hydrophobic and reducing the repulsive forces between

p

mi

4.15 ± 0.25 4.34 ± 0.31 1.68 ± 0.60 1.77 ± 0.93

hem thus leading to increased biosorption [17]. The increasediosorption in the presence of calcium was greater than that ofagnesium in present study.

.4.2.3. Destabilization and precipitation. Hydrolyzing metal-ic cations, such as Al3+ and Fe3+, undergo hydrolytic reactionsn aqueous solution to form monomeric, multimeric, and/orolymeric species, or aluminium (ferrum) hydroxide precipitatehen the aluminium (ferrum) concentration is supersaturatedith respect to the solubility of amorphous aluminium (ferrum)ydroxide [24,25]. Solution chemistry exerts a significant influ-nce on the hydrolyzing reactions and precipitation, and thus onhe coagulation behaviour [26–28]. It was generally observedhat the optimum concentration of hydrolyzing metallic salts wasround 10−2 mol/L for humic substance removal in biosorptionn the study.

When hydrolyzing metallic salt coagulants are added to theA and activated sludge biosorption system, several processesay occur and there may be competition between them. HA in

olution may firstly react with the metallic ion hydrolysis specieso form either soluble complexes, which may be then adsorbednto the activated sludge surfaces, or insoluble precipitates thatemain suspended in the bulk solution. Alternatively, HA maye directly adsorbed onto the surface of activated sludge beforenteracting with the coagulant hydrolysis species. Besides, manyther types of interactions may also occur. The overall effectill depend on the proportion of activated sludge and coagulant,

nd the relative kinetics of the interactions between the HA, thectivated sludge and the coagulant. Furthermore, since the HAs a heterogeneous matrix of organic species, it is likely that theinetics of interaction will vary in relation to the number and

roperties of the individual humic fractions [9].

At a dose of 10−3 mol/L, the interaction between the alu-inium (ferrum) hydrolysis species and the HA can cause an

ncrease in the overall HA removal compared with the adsorp-

4 ginee

tscttttahoidh

4

tbrttpAooao

cbwtHwicthmartvbapch

A

S(to

R

[

[

[

[

[

[

[

[

[

[

[

[

[

[

84 H.-J. Feng et al. / Biochemical En

ion alone. It is speculated that the aluminium (ferrum) dose istill insufficient to cause significant HA destabilization and pre-ipitation, but the interaction of aluminium (ferrum) species withhe HA led to the adsorption which was still better than biosorp-ion alone. At the aluminium (ferrum) dose of 10−2 mol/L,here was most likely an extensive precipitation of HA frac-ions, which thereby substantially increased the extent of HAdsorption; leading to an overall increased HA removal. At theigher coagulant doses of 10−1 mol/L, the Al3+ and Fe3+ areversaturated with respect to amorphous aluminium hydrox-de and ferric hydroxide, which make the colloid charge andestabilization. On the contrary, the excessive concentration ofydrolyzing metallic salts led to a diminished adsorption.

. Conclusions

Activated sludge was successfully applied as biosorbento remove humic substances from aqueous medium. It cane confirmed that biosorption instead of biodegradation, wasesponsible for HA removal. However, the mechanisms con-rolling biosorption were not well understood. It was criticalo work with activated biomass, rather than dried biomass, toreserve the integrity of surface characteristics of the biosolids.lthough the Freundlich isotherm model fitted the biosorptionf HA by activated sludge biomass, it was necessary to pointut that biosorption of HA corresponded to the biosorption ofmixture of adsorbates because of the heterogeneous characterf these substances.

The effects of performance parameters such as initial HAoncentration, adsorbent dosage and inorganic metallic ions oniosoption were analyzed. The removal efficiency improvedith the enhancing initial HA concentration. While biosorp-

ion of HA can suitably be applied to treat high concentrationA wastewater because the HA concentration of blank sampleas approximate to 1 mg/L. The total HA removal efficiency

mproved with the increasing adsorbent (activated sludge) con-entrations. As far as engineering application was concerned,he shorter contact time of pseudo-equilibrium was desirable, theigher ratio of the adsorbent and adsorbate (HA) may be recom-ended. Furthermore, biosorption of HA was favored through

dding inorganic metallic cations. The mechanism of HAemoval was variable after adding different cations. Increasinghe ionic strength had a positive effect on HA bisorption by acti-ated sludge biomass for salt (sodium). Acting as cationic bridgeetween HA and activated sludge, divalent cations (magnesiumnd calcium) favored biosorption of HA. Destabilization andrecipitation increased HA bisorption for hydrolyzing metallication (aluminium and ferrum) while excessive concentration ofydrolyzing metallic salts led to a diminished adsorption.

cknowledgements

This research was the Key Project supported by the

cience and Technology Department of Zhejiang Province2005C13003). We all really appreciate J.L. Liu for providingechnical assistances, and will also acknowledge the reviewersf this article.

[

[

ring Journal 39 (2008) 478–485

eferences

[1] S. Bratskaya, S. Schwarz, D. Chervonetsky, Comparative study of humicacids flocculation with chitosan hydrochloride and chitosan glutamate,Water Res. 38 (2004) 2955–2961.

[2] B.C. Qi, C. Aldrich, L. Lorenzen, Effect of ultrasonication on the humicacids extracted from lignocellulose substrate decomposed by anaerobicdigestion, Chem. Eng. J. 98 (2004) 153–163.

[3] T. Moriguchi, K. Yano, M. Tahara, K. Yaguchi, Metal-modified silica adsor-bents for removal of humic substances in water, J. Colloid Interface Sci.283 (2005) 300–310.

[4] W. Andayani, A.N.M. Bagyo, Radiation-induced decomposition and decol-orization of water in aerated solutions, Radiat. Phys. Chem. 65 (2002)447–450.

[5] Y. Seida, Y. Nakano, Removal of humic substances by layered doublehydroxide containing iron, Water Res. 34 (2000) 1487–1494.

[6] R. Bian, Y. Watanabe, N. Tambo, G. Ozawa, Removal of humic substancesby uf and nf membrane systems, Water Sci. Technol. 40 (1999) 121–129.

[7] J. Duan, F. Wilson, N. Graham, J.H. Tay, Adsorption of humic acid bypowdered activated carbon in saline water conditions, Desalination 151(2003) 53–66.

[8] H.S. Wu, A.Q. Zhang, L.S. Wang, Immobilization study of biosorption ofheavy metal ions onto activated sludge, J. Environ. Sci. 16 (2004) 640–645.

[9] M. Esparza-Soto, P. Westerhoff, Biosorption of humic and fulvic acids tolive activated sludge biomass, Water Res. 37 (2003) 2301–2310.

10] T.G. Egil, The role of humic substances in the acidification response of soiland water: results of the humic Lake acidification experiment(Humex),Environ. Int. 20 (1994) 363–368.

11] S.B. Choi, Y.S. Yun, Biosorption of cadmium by various types of driedsludge: an equilibrium study and investigation of mechanisms, J. Hazard.Mater. 138 (2006) 378–383.

12] M. Fukushima, K. Tatsumi, K. Morimoto, Influence of iron(III) and humicacid on the photodegradation of pentachlorophenol, Environ. Toxicol.Chem. 19 (2000) 1711–1716.

13] E.M. Thurman, R.L. Malcolm, Preparative isolation of aquatic humic sub-stances, Environ. Sci. Technol. 15 (1981) 463–466.

14] J.L. Huang, Z.Y. Bao, Ultraviolet Absorption Spectroscopy and its Applica-tion, China Science & Technology Press, Beijing, China, 1992 (in Chinese).

15] D.J. Barker, S.M.L. Salvi, A.A.M. Langenhoff, D.C. Stuckey, Solublemicrobial products in ABR treating low-strength wastewater, J. Environ.Eng. 126 (2000) 239–249.

16] J.L. Zhou, Biosorption and desorption of humic acid by microbial biomass,Chemosphere 24 (1992) 1573–1589.

17] J.L. Zhou, C.J. Banks, Mechanism of humic acid colour removal fromnatural waters by fungal biomass biosorption, Chemosphere 27 (1993)607–620.

18] J.J. McCreary, V.L. Snoeyink, Characterization and activated carbonadsorption of several humic substances, Water Res. 14 (1980) 151–160.

19] Z.W. Zhao, Y.P. Xu, Effect of removal efficiency of NOM parameter bycontrolling coagulation condition, J. Harbin Univ. Comm. Nat. Sci. Ed. 19(2003) 191–193 (in Chinese).

20] S. Srisurichan, R. Jiraratananon, A.G. Fane, Humic acid fouling in themembrane distillation process, Desalination 174 (2005) 63–72.

21] B.W. Derjaugin, L. Landau, Theory of the stability of strongly chargedlyophobic sols and the adhesion of strongly charged particles in solutionsof electrolytes, Acta Physiochim. (USSR) 14 (1941) 633–662.

22] A. Zita, M. Hermansson, Effects of ionic strength on bacterial adhesion andstability of flocs in a wastewater activated sludge system, Appl. Environ.Microbiol. 60 (1994) 3041–3048.

23] W.J. Weber Jr., C.V. Thomas, J. Abdullah, Adsorption of humic substances:the effects of heterogeneity and system characteristics, J. Am. Water Works

Assoc. 75 (1983) 612–619.

24] S.K. Dentel, J.M. Gossett, Mechanisms of coagulation with aluminum salts,J. Am. Water Works Assoc. 80 (1988) 187–198.

25] H.W. Ching, M. Elimelech, G.J. Hering, Dynamics of coagulation of clayparticles with aluminum sulfate, J. Environ. Eng. 120 (1994) 168–189.

ginee

[

[

H.-J. Feng et al. / Biochemical En

26] N. Calace, B.M. Petronio, S. Persia, M. Pietroletti, D. Pacioni, A newanalytical approach for humin determination in sediments and soils, Talanta71 (2007) 1444–1448.

27] J.Q. Jiang, N.J.D. Graham, C. Harward, Comparison of polyferric suly-ferric sulfate with other coagulant for the removal of algae and

[

ring Journal 39 (2008) 478–485 485

algae-derived organic matter, Water Sci. Technol. 27 (1993) 221–230.

28] J. Duan, J. Gregory, The influence of silicic acid on aluminium hydroxideprecipitation and flocculation by aluminium salts, J. Inorg. Biochem. 69(1998) 193–201.