Removal of various heavy metals by sewage sludge immobilized onto rod- 2 chitosan biosorbent

22 The potential use of wastewater sludge as a biosorbent for removing various metals and 23 metalloids from aqueous solutions was examined. The sludge was immobilized by chitosan 24 into rods to enhance the sorption capacity and solid-liquid separation ability. An optimum 25 condition of rod-type chitosan-immobilized sludge (RCS) was selected from possibility of 26 produced biosorbent and the removal efficiency of As(V). The optimal sludge and chitosan 27 content and RCS thickness was 6.0%, 4.0% and 0.2-0.3 mm, respectively. The experiments 28 targeted cations(Cd(II)) and anions(As(V), Cr(VI), and Mn(VII)) . Pseudo-first-order and 29 pseudo-second-order models adequately described kinetics models and Langmuir and 30 Freundlich models described isotherm models for RCS, which showed higher adsorption 31 ability for anionic metals over cationic metals. The results indicate that electrostatic attraction 32 or ion exchange is the most important mechanism for metal/metalloid adsorption , except i n 33 the case of Mn(VII), for which an adsorption-coupled reduction mechanism i s suggested. 34 an adsorption-coupled


Introduction
were performed in triplicate, with an error rate of less than 5%. Mean values were used for 115 kinetic and isotherm experimental data. 116 117

Analytical methods 118
TOC of the solution was measured using a TOC analyzer (TOC-VCPH/CPN, 119 SHIMADZU, Japan). Total nitrogen (TN) and total phosphorous (TP) were determined using 120 a test kit (C-MAC. Co., Korea). An infrared spectrum of the biosorbent with an FTIR 121 spectrometer (Vertex 70, Bruker, USA). An inductively coupled plasma-optical emission 122 spectrometer (ICP/IRIS, Thermo Jarrell Ash Co., USA) was used to analyze total metals after 123 being filtered through a 0.20 µm membrane. The Cr(VI) concentration was determined by 124 spectrophotometric analysis at 540 nm according to a standard method using 1,5-125 less eluted (Table 1), suggesting that immobilization can alter the properties of a biosorbent, 141 including surface functional groups. In particular, a fuctional groups on the surface often 142 changes after immobilization. To determine the nature of functional group change, raw 143 biosorbents and chitosan-immobilized biosorbents were examined using FTIR analysis (SI 144 Fig. 1). The spectra of the raw biosorbent and chitosan-immobilized biosorbents displayed 145 many absorption peaks. A broad and strong band in the 3600-3200 cm − showed some peak changes (SI Fig. 1). The band at 3378 cm −1 broadened because of the 152 large number of amine groups in chitosan. Amine group biosorption sites played the most 153 significant role in anionic metal removal [16]. The FTIR results show that chitosan-154 immobilized biomass was expected to remove anionic metal more effectively than raw 155 biomass [17]. 156 sludge (BCS), and rod-type chitosan-immobilized sludge (RCS) were performed to evaluate 165 the contact time required to reach equilibrium. Chitosan is a biopolymer with a high nitrogen 166 content that confers an adsorption ability for anionic metal ions. As(V) was used to evaluate 167 the performance of each biosorbent. Figure 1

Effect of material content in biosorbent 173
The amounts of sludge and chitosan were important variables affecting sorption 174 performance. Figure 1(b) shows the effects of sludge on As(V) removal onto RCS. The 175 sludge content influenced the biomass surface, affecting the capacity of the biomass for 176 As(V) biosorption. For finer details, we compared adsorption results through pseudo-first-177 order and pseudo-second-order kinetics modeling. These models were employed to 178 investigate the adsorption dynamics of pollutants onto the biosorbents in relation to time and 179 to estimate the rate of the process. They can also shed light on biosorption mechanisms and 180 potential rate-controlling steps, which may include mass transport and chemical reaction 181 processes [19]. The pseudo-first-order equation is: 182 where qe is the amount of adsorbate adsorbed (mg/g) at equilibrium, qt is the amount of 186 adsorbate adsorbed (mg/g) at time t (min) and k1 (min -1 ) is the rate constant. 187 The pseudo-second-order equation is usually associated with the situation in which the 188 rate of direct adsorption or desorption controls the overall sorption kinetics, and typically describes the removal behavior of metals [20,21]. An integrated form of the pseudo-second-190 order equation can be expressed as: 191 were examined, the effect of chitosan in RCS was not as dramatic, considering the 217 economical manufacture of biosorbent focuses on RCS with sludge and chitosan contents of 218 6.0% and 4.0%, respectively. 219 220

Effect of biosobent size 221
Biosorbent diameter size is an important controlling parameter of the biosorption 222 process. The effect of RCS diameter size on As(V) biosorption was studied using samples of 223 four biosorbents with average diameters of 0.2-0.3, 0.5-0.6, 0.8-0.9 and 1.0-1.2 mm, and 224 sludge and chitosan contents of 6.0% and 4.0%. The results are presented in Fig. 1

(d). 225
Equilibrium uptake values showed similar values (Table 2). However, the rate constant and 226 initial sorption rate decreased when the biosorbent diameter increased from 0.2-0.3 to 1.0-227 1.2 mm. The higher biosorption with smaller RCS diameters may be attributed to the fact that 228 RCS with smaller diameter has greater surface area. Therefore, an RCS diameter size of 0.2-229 0.3 mm was selected for experimental purposes.  Table 3 shows that the pseudo-second-order equation, which agrees that chemisorption is the rate-controlling mechanism, was able to better describe the adsorption of As(V), Cd(II), 239 Cr(VI) and Mn(VII) onto RCS. By comparing the amount of metals adsorbed at equilibrium, 240 the following order was obtained: Mn(VII) > Cr(VI) > As(V) > Cd(II). While Mn(VII) was 241 completely removed, sorption of Mn(VII) did not fit the kinetics models. The Mn(VII) 242 removal mechanism is recognized to differ from that of other metals. To examine the 243 Mn(VII) removal characteristics of RCS, total Mn concentrations were investigated (SI Fig.  244 2). After complete Mn(VII) removal, Mn remained in the aqueous phase. It can therefore be 245   (Table 4). The other metals, Cd(II) and Mn(VII), were well described by the Freundlich 282 equation based on R 2 . This difference may be explained by the presence of an operating 283 mechanism other than basic ion exchange, such as specific adsorption-complexation reactions 284 taking place in the adsorption process [28]. As mentioned above, the biosorption mechanism 285 of Mn(VII) was suggested by an adsorption-coupled reduction mechanism. 286

288
The aim of the present study was to develop a high-performance biosorbent of non-289 living activated sludge. The incorporation of immobilizing chitosan, which was confirmed by 290 the release of TOC, TN, and TP from the biosorbent during pretreatment with deionized-291 distilled water wash, may offer an effective method of decreasing metal and metalloid 292 concentrations in wastewater. Comparing experiments for adsorption of As(V), the optimal 293 conditions for biosorption were a rod shape, 4.0% chitosan, and a biosorbent diameter of 0.    Table 3. Kinetic and isotherm constants for the biosorption of As(V), Cd(II), Cr(VI) and Table 4. Maximum uptakes of As(V) by various biosorbents manufactured from sludge 499