当前World Environment

Introduction

重金属废水已成为focus of environment remediation efforts because of their toxicity and threat to industrialization and urbanization with new technology. Wastewater containing Ni, Cr, Fe originate from primarily metal industry particularly during plating, tannery operation. But their concentration and presence highly depends upon the type of method used for like ion-exchange, reverse osmosis, electrodialysis are costly and at the same time, methods like chemical precipitation, coagulation, flocculation etc. are less effective and has a disadvantage of sludge formation¹. However, such method is not preferred since sludge disposal problem. Adsorption is a sludge free process and had unergone many improvements. Besides these methods, now a day many scientists are attracted towards the adsorption is the process by which a solid adsorbent can attract a component from the aqueous phase to its surface and there by form an attachment via a physical or chemical bond, thus removing the component from the aqueous phase. Adsorption, which has been preferred over other process because of its cheapness and the high-quality treated effluent it produces.

Adsorption of Cr from Industrial effluents using agricultural waste², Ni from electroplating Industry by agro waste carbon,³ the removal Ni ions in aqueous media using low cost adsorbent,⁴the adsorption of Fe using low cost materials,⁵the adsorption of Fe, Ni, Cr using maize cob carbon.⁶Many reports have been appeared on the development of low-cost activated carbon adsorbent from cheaper and readily available material^7-9their high surface areas, microporus characters and surface chemical natures have made such activated carbons potential adsorbent for the removal of heavy metals from industrial wastewater. Among the several methods, adsorption is highly effective and comparatively cheap, hence in present investigation to develop inexpensive and effective metal ions (Cr⁺⁶, Fe⁺²and Ni⁺²) adsorbents from plentiful sources of natural wastes (or by-products) such as tree leaves (Ber, Neem, Ruchkin, Teak and Tea-waste) and its activated carbon to offer these adsorbents as replacements for existing commercial materials.

Material and Methods

Preparation of Adsorbent

The leaves ofTeak(Verbenaceae),Ber(Zizipus amaranthus),Neem(Azardicita indica),Ruchkin(Calotropis) from twigs and Tea-waste fromcanteen were gathered into clean plastic bags. Washed with distil water and laid flat on clean table on dry. Dry leaves were grounded with mixer. After grounded, the leaf particle were sieved and stored into plastic bag is sieved size (0.60 - 1.60 mm), and ready for use.

Table 1: Different tree leave species (fresh) as adsorbent for metal ions from Synthesis wastewater Click here to view table

Chemical activation using H₂SO₄at moderate temperature produces a high surface area and high degree of micro-pososity.¹⁰The materials were mixed in 1:1 weight ratio with concentration H₂SO₄and allowed to soak for 24h at a room temperature. The samples were placed in an over and heated at 200°C. Where they were held for 24 hours after this the sample was allow to cool back to room temperature. Then the samples were washed with distilled water and soaked in 1% NaHCO₃solution to remove any remaining acid. The samples were washed with the distilled water until pH of the activated carbon reached 6, dried at 105°C for 5 hours.

Figure 1: Effect of contact time for the removal of Cr (VI) by tree leaves Click here to view figure

Synthetic Wastewater Preparation

All the chemicals were of analytical grade (Qualigens), Colorimeter (photoelectric AE-11m19942) were used determination of Cr, Fe and Ni, metal ions. The wastewater containing Cr, Fe and Ni was prepared in the laboratory by dissolving a known amount of salt i.e Cr, Fe and Ni in distilled water. All three solutions prepared about 20 ppm concentrated solution. The salt used as K₂Cr₂O₇-7H₂O, NiSO₄-6H₂O and FeSO₄7H₂O.

Figure 2: Effect of contact time for the removal of Cr (VI) by activated tree leaves Click here to view figure

To understand the adsorption behavior a number of batch studies carried out like pH and contact time at constant of adsorbent dose for these studies. We prepare wastewater of various concentrtions of Cr, Fe and Ni. From stock solution of 20 ppm concentration and kept separately in a bottle then suitable dose (2gm) were added to the 200ml of wastewater. Shaking at 300 rpm for 2 to 3 hours at room temperature, so adequate contact between adsorbent and metal ions has maintained, equilibrates the system. The suspension is filter by centrifugation for 10 min at 4000 rpm and filtrate is analyzing to evaluate the final concentration of Cr, Fe, Ni metalss in treated wastewater by using calorimeter (Photoelectric calorimeter AE-11m, 99942). All the analysis carried out by standard method.¹¹

Figure 3: Effect of contact time for the removal of Fe by tree leaves Click here to view figure

Results and Discussion

Adsorption on Tree Leaves (Fresh)

Table 1 represents the adsorption of three kinds of metal ions on five kinds of tree leaves. It reveals that different metal ion on the same tree leaf had different removal rate. At the same experimental conditions (Fe⁺²) and Cr (Cr⁺⁶) had highest removal rate. Ni⁺²had lowest removal rate exceptTea-waste. It has been observed that the highest removal rate for Cr+6 (67%), Fe⁺²(70%) and Ni⁺²(63%).

Table 2: Different tree leave species (activated carbon) as adsorbent for metal ions from Synthesis wastewater Click here to view table

For the same metal ion, different tree leaves had different removal rates. In the tested leaf groupNeem(67%) andBer(64%) showed highest removal rates for chromium (Cr⁺⁶).Ber(70%) and Neem (67%) also showed higher removal rate for ions (Fe⁺²). The highest removal rates for Nickel ion (Ni⁺²)Tea-waste(63%) andTeakleave species (58%).

Adsorption Experiments

The experiments were carried in the batch mode for the measurements of adsorption capacities. The bottles with 250mL capacity were filled with wastewaer and adsorbent. The bottles were shaken for a predetermined period at room temperature in a reciprocating shaker. The separation of the adsorbents and solution was carried out by centrifugation. The solution were analyzed by using calorimeter.

Figure 4: Effect of contact time for the removal of Fe by activated tree leaves Click here to view figure

Adsorption on activated carbon of tree leaves

Table 2 also showed the results of adsorption of metal ion, Cr+6, Fe+2 and Ni2+ on the five kinds of activated carbon of tree leave species. The experiments were conducted under the same conditions as in the adsorption by tree leaves. The experimental results are tabulated in Table 2. It reveals that at the same experimental conditions (Cr⁺⁶), (Fe⁺²) and (Ni⁺²) had highest removal rate Cr⁺⁶(85%), Fe⁺²(76%) and Ni²⁺(72%).

In the tested activated carbon of leaf groupNeemshowed highest removal rate for Cr⁺⁶,NeemandBershowed highest removal rate for Fe⁺²and the highest removal rate for Ni²⁺ion Tea-waste.

Figure 5: Effect of contact time for the removal of Ni by tree leaves Click here to view figure

In comparison with the adsorption of metal ions from wastewater by activated tree leaves, did not showed supreme results. The best removal rate for Cr (VI) was by activatedNeemleaves (85%) which are higher thanNeemleaves (67%) under the same test conditions. ActivatedBerremoved (70%) andTea-wasteremoved (67%) of Cr (VI), which is close results as tree leaves. i.e. (64%) and (63%) respectively. The best removal rate for Fe²⁺was by activatedBerandNeem(76%) and activatedRuchkin(75%) which is higher thanBertree leaves (70%),Neemtree leaves (67%) andRuchkin(55%). The best removal rate for Ni²⁺activatedTea-waste(72%) while other studied activated species were in the range (65-67%), which is higher than tree leave species.

Figure 6: Effect of contact time for the removal of Ni by activated tree leaves Click here to view figure

Influence of Contact Time on Adsorption

Preliminary experiments showed that teh adsorption of metal ions by leves reached equilibrium in more than 100 minutes (Fig. 1-6). The experimental resuls of kinetic study of chromium ion (Figs. 1-2), iron (Figs. 3-4) and nickel ion (Figs. 5-6) from synthetic waste water containing 20 ppm metal ions withBer, Teak, Ruchkin, Neem, Tea-wasterespectivley. The experiments were carried out under conidition of room temperature, with 2 gm of 0.60 mm to 1.60 mm leaf size in 200 ml of synthetic wastewater. The initial pH for synthetic wastewater was about 6. (Figs. 1-6). Reveals that the percentage of removal metal ions (Cr⁺⁶, Fe⁺²and Ni²⁺) increases with an increase contact time at fixed absorption dose and concentration. Fig. 1 showed that the contact time used for all studied metal ions reveals that negative deviation byTeak叶子高达40分钟。这是因为due to small traces of chromium content in the leaves. The expeiments showed that hte removal rate occur quickly, seemly reaching equilibrium within the first hundred minutes of adsorption.

Conclusion

1. Tree leaves can be used in the wastewater treatment process for the removal of metal ions.
2. The adsorption of the metal ions on tree leaves reached equilibrium more than 100 minutes.
3. The adsorption of different metal ions on the same tree leaf had different removal rate,chromium (Cr⁺⁶) had the highest removal rate, nickel (Ni⁺⁺) had lowest removal rate.
4. For the same metal ion, different tree leaves had different removal rateBerandNeem显示为铬去除率最高(Cr⁺⁶).Tea-wasteshowed the highest removals rates for Nickel ion (Ni⁺⁺) andNeemandBerleaves showed that highest removal rate for Iron (Fe⁺⁺).

References