Fruit/vegetable wastesAdsorption of

Fruit/vegetable wastes

Adsorption of divalent heavy metal ions particularly Cu2+, Zn2+, Co2+, Ni2+ and Pb2+ onto acid and alkali treated banana and orange peels was performed by Annadurai et al. (2002). The acid and alkali solutions used for modification of adsorbents were HNO3 and NaOH. In general, the adsorption capacity decreases in the order of Pb2+ > Ni2+ > Zn2+ > Cu2+ > Co2+ for both adsorbents. Banana peel exhibits higher maximum adsorption capacity for heavy metals compared to orange peel. The reported maximum adsorption capacities were 7.97 (Pb2+), 6.88 (Ni2+), 5.80 (Zn2+), 4.75 (Cu2+) and 2.55 mg g−1 (Co2+) using banana peel; and were 7.75 (Pb2+), 6.01 (Ni2+), 5.25 (Zn2+), 3.65 (Cu2+) and 1.82 mg g−1 (Co2+) using orange peel. Acid treated peels showed better adsorption capacities followed by alkali and water treated peels. Based on regeneration studies, it was reported that the peels could be used for two regenerations for removal and recovery of heavy metal ions.

Besides NaOH, Ca(OH)2 is another good saponifying agent for the conversion of ester groups to carboxyl groups as demonstrated by Dhakal et al. (2005). In the study, orange waste (consists of cellulose, hemicellulose, pectin, limonene and other low molecular weight compounds) was treated with Ca(OH)2 to form saponified gel (SOW). Two forms of saponified gels were prepared (Ca2+-form and H+-form) and their removal efficiency for six heavy metal ions particularly Fe(III), Pb(II), Cu(II), Zn(II), Cd(II) and Mn(II) were compared. The authors suggested that cation exchange was the main mechanism for the removal of heavy metal ions as the pH of solutions decreased after adsorption. The order of removal for Ca2+-form SOW gel was Pb(II) > Fe(III) > Cu(II) > Cd(II) > Zn(II) > Mn(II). In the case of H+-form SOW gel, the order of removal was Pb(II) > Fe(III) > Cu(II) > Zn(II) > Cd(II) > Mn(II). As the pH of solutions increases, the percent removal of heavy metal ions also increased except Fe(III). The percent removal of Fe(III) greatly reduced beyond pH 3 due to formation of soluble iron complexes such as Fe(OH)+, View the MathML source, View the MathML source and View the MathML source. The authors also suggested that ion-exchange mechanism involves oxygen atom in the pyranose ring of pectin acids cooperating with carboxylic group to form a stable five-membered chelate ring. This study indicates that both types of SOW gels are effective for removing heavy metal ions in acidic solution.Chemical modification of cornelian cherry, apricot stone and almond shell by using concentrated sulfuric acid for the removal of Cr(VI) has been studied (Demirbas et al., 2004). All the three types of fruit wastes showed highest removal of Cr(VI) at pH 1. It was also reported that adsorption was highly dependent on the initial metal concentration as the lowest concentration recorded fastest removal rate (shortest equilibrium time). The equilibrium time for cornelian cherry was 20 h at 53 mg l−1 Cr(VI) concentration, increased to 70 h as the concentration increased to 203 mg l−1. The removal rate increased with a decrease in adsorbent size, which indicates that smaller particle size has larger surface area. Four different kinetic models particularly pseudo-first-order, pseudo-second-order, Elovich and intraparticle diffusion were evaluated and results showed that pseudo-second-order model correlated well with the experimental data. Recently, Kula et al. (2007) reported the application of ZnCl2 (a dehydrating agent) in the activation of olive stone for removal of Cd(II) ions. It was reported that treated olive stone shows a remarkable increase in surface area compared to untreated olive stone. However, the activated olive stone did not show good adsorption capacity for Cd(II) as the reported maximum adsorption capacity was only 1.85 mg g−1.

Li et al. (2006b) investigated orange peels as an adsorbent for cadmium adsorption and the effect of different citric acid concentrations on the adsorbent characters was studied. Upon treatment with more concentrated citric acid solutions, orange peels showed lower values of pH of zero point charge (pHzpc) due to the increase number of total acidic sites while the total number of basic sites decreased. The increase in citric acid concentration results in more oxygenated groups being introduced to the adsorbent surface. Orange peels washed with 0.6 M citric acid at 80 °C has a much lower pHZPC value indicating that the adsorbent surface becomes more negative due to dissociation of weakly acidic oxygen-containing groups. Chemical treatment with citric acid at high temperature produced condensation product and citric acid anhydride. The reactive citric acid anhydride can react with cellulosic hydroxyl groups to form an ester linkage and introduce carboxyl groups to the cellulose (Marshall et al., 1999). The presence of more carboxyl groups will increase more cadmium ions to bind on the adsorbent surface. It was also reported that

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wastes / vegetable水果二价金属离子的金属离子对重金属Adsorption particularly Cu2 +，CO2 +，Ni2 Zn2 +，+ +和Pb2 treated香蕉和onto acid alkali橘子皮是由Annadurai和performed等人。2002）和解决方案为：用酸改adsorbents of alkali一般是在物理和NaOH，和吸附容量。decreases order of Pb2在Ni2 + + > > > > Zn2 +，CO2 +和Cu2 +两高等adsorbents陈皮。香蕉对重金属吸附容量最大，exhibits相比），陈皮metals reported橙色。最大吸附Pb2 capacities是7.97（+），（+），5.80 6.88 Ni2（+），4.75 Zn2 2.55（Cu2 +）和Mg（g）+ CO2 - 1使用香蕉是7.75 Pb2陈皮；和（+），（+），5.25 6.01 Ni2（+），3.65 Zn2 1.82（Cu2 +）和Mg（g）+ CO2 - 1使用treated陈皮酸橙色。通过更好的吸附遵循capacities showed皮皮。treated alkali和再生水为基础，研究它在这是一个reported皮再生，可能是用两个金属的回收率，对重金属离子removal和。此外，NaOH、Ca（OH）2皂化剂，是一个很好的carboxyl酯组到组的转换，表明As通过Dhakal等人的研究。在2005），橙色（包括纤维素，半纤维素的废物和其他低，果胶，limonene化合物的分子量是treated）与Ca（OH）2到SOW形式saponified凝胶（凝胶）。二是prepared saponified forms of（Ca2+-form H+-form）和他们的效率和对重金属离子和六removal particularly金属Fe（III），铅（II），铜（II），锌（II），镉（II）和Mn（II）的相比，这是suggested authors .）为阳离子交换的机制，是对重金属的removal of mainpH值的解决方案作为一个金属离子）吸附后的产量下降。removal顺序为Ca2+-form SOW凝胶是铅（II）> Fe（III）铜（II）> > > Cd（II），锌（II）-锰（II）的情况。在K的顺序H+-form SOW凝胶，removal是铅（II）的Fe（III）> > Cu（II）锌（II）> > > Cd（II）-锰（II）的解决方案。为提高pH值，金属离子对重金属也removal percent of increased除的Fe（III）的百分之。removal），大大减少的Fe（III）的pH值超出3由于形成可溶性复合物，如铁和As，Fe（OH）+查看的源，源的MathML的视图View source和MathML。suggested MathML），这也ion-exchange authors氧原子在pyranose机制involves果胶与羧酸环acids合作的形式稳定的five-membered A组这是chelate环B。这两indicates SOW凝胶是有效的金属离子对重金属在酸性和removing solution.Chemical樱桃，杏石改的红宝石和almond壳使用的酸，由concentrated sulfuric removal ofCr（VI）已被研究2004 Demirbas et al .，（三）（B）。所有的水果showed wastes最高的Cr（VI）在removal这是reported pH = 1。这也是高度依赖的初始吸附在金属浓度作为一个lowest浓度recorded fastest removal shortest equilibrium率（时间）时间）。equilibrium樱桃红宝石是在20 H 53 mg L - 1Cr（VI）的浓度，浓度对70 H作为一个increased increased mg L - 1到203 increased）与一个removal率。吸附降低，这是在indicates size大小有更大（smaller转录动力学模型四个不同表面区域。particularly pseudo-first-order，pseudo-second-order Elovich和评估，结果是intraparticle showed和扩散那好pseudo-second-order模型与实验数据相关。Recently Kula等人，reported 2007）的应用ZnCl2脱水剂：（a）在激活之后，removal橄榄的Cd（II）离子（这是treated reported那。在一个特别对橄榄石表面凝集增长区相比，未经处理的橄榄橄榄石。然而，活性做不好之后，吸附容量：镉（II）的最大吸附容量为reported不仅是1.85 mg g - 1。李等人的研究（2006b橘子皮吸附），吸附作为一个cadmium和柠檬酸的不同效应。在研究的浓度，是治疗你。Upon吸附柠檬酸的解决方案，提供更多的concentrated，橙色showed values of皮下的零点（pH值）由于在pHzpc增长到总在酸性的网站数number of总产量下降。在基本的网站-柠檬酸浓度的增长。更重要的是在组织氧合吸附到表面。引进0.6橙皮与柠檬酸洗.在有多80°C下indicating pHZPC a值更重要的是由于吸附的表面becomes型弱酸性的含氧组分对dissociation化工。在高处理温度与柠檬酸和柠檬酸，产品produced凝聚反应。对柠檬酸酐酸与顺丁烯二酸酐可以react纤维素酯和hydroxyl组到一组连锁介绍到carboxyl和纤维素。1999（马歇尔等人，就更carboxyl（）。），更将增长到cadmium组离子吸附在表面绑定。这是reporte ]