The compounds isolated from natural raw materials having the coagulation inducing activity can be considered as the natural coagulants. Natural coagulants are, most often, isolated from plants and their chemicals structures can be very diverse. The investigations have shown that the natural coagulants also can be obtained from microorganisms and from animal tissues, as well [Kawamura, 1991]. Raw materials of animal origin have not been investigated sufficiently yet, while the application of natural coagulants still lies in its initial phase, and additionally, the processes necessary for the processing of these materials are costly and complicated. For example microbiological processes create good protein yields but they must be accompanied with the additional purification steps, making such procedure more expensive compared to the one based on the raw materials of plan origin. The basic advantage of natural coagulants represents the fact that it leaves the biodegradable residue that can be easily processed and disposed [Ozacar et al., 2003]. A number of plant materials containing potentially the coagulation stimulating substances, such as bark and core of the horse chestnut, acorns, acacia, locust bean, common bean, soybean, etc. [Stojimirović, 2005; Homa, 2006; Beljkaš, 2007]. Until now, the best examined is extract obtained from seeds of Moringa oleifera, which showed the highest effects as the primary coagulant and which can be compared with aluminum sulphate, the most often used coagulant for purification of water [Berger et al., 1984]. Berger et al., [1984] concluded that active components from M. oleifera responsible for the clarification of water are proteins. At concentrations used for the water clarification, this coagulation agent is not dangerous in respect to the human health, quite opposite to the aluminium-containing compounds which are considered to induce the Alzheimer’sisease [Mallevialleet al.,1984].

Artificial turbid water was applied as the model water for the determination of coagulation activity of the partially purified extract of proteins from the bean seeds. The model water was obtained by the dilution of 15 ml suspension of kaolin with the tap water immediately before the coagulation experiments, using the addition of 5 mL of the suspension per 1 L of water. So prepared model water had the turbidity of 35 NTU. Such model water was chosen because of the comparability with the data from the literature, while the experiments were mostly performed with model water turbidified with kaolin.

The preparation of natural coagulants has been done from the seed of white beans obtained from the Institute of field and vegetable crops of the Faculty of agronomy, Novi Sad. White bean seeds were grounded and sieved with the sieve with light area width of 0.4 mm, then suspended in the distilled water at the concentration of 10 g/L, with the intensive mixing with magnetic stirrer for 10 minutes in order to improve the extraction. So obtained suspension was filtered, using the widepore filter paper, in order to remove solid residues.

Water turbidity testing, after addition of the coagulant and the finished settling, was performed by the nephelometric method, using the haze meter VOS 4000, according to the instruction of the manufacturer. The obtained values for turbidity were expressed in NTU units (nephelometric turbidity unit). pH values were determined by the pH-meter HI 9321.

Immediately before the experiment, the model water was prepared by the dilution of the kaolin suspension with the tap water. In the so obtained model water, on the magnetic stirrer, pH value was adjusted to the desired value, using the additions of the solutions of NaOH (1 Mol/L) or HCl (1 mol/L). Model water was divided into four beakers, 300 mL per beaker, which were introduced into the mashing bath Glasbläserei (water bath with the mixer) and mixed with the higher mixing speed (200 rpm) for 1 or 2 minutes. At the very beginning of fast mixing, in glasses 0.9 mL of the coagulant suspension was added, corresponding to the extract from 30 mg of beans per 1 L of the model water. After that, mixing rate was decreased to 80 rpm. Slow mixing duration was 15, 30 or 60 minutes.

After finishing the mixing, metal beakers were taken out from the apparatus; their contents were transferred into glass beakers, where they were left for 1 hour in order to settle. The blank assay was the model water that was also poured into the beaker, and left to settle for 1 hour. After that, 100 mL of the clarified supernatants (upper layer) were pipetted off and their turbidities were measured. The coagulation activity was calculated with the application of the following formula

where: CA = coagulation activity; Tba = turbidity of the blank assay; Ts = turbidity of a sample.

Colloid particles that induce the turbidity are so hydrated that their density approximately equals to the density of water. Owing to that, their removal by sedimentation is very slow or impossible. Besides that, the existence of charge on the particles prevents them joining. At the isoelectric point the particles are electrically neutral, so they join with each other and settle to the bottom. With increasing distance of the pH value from the isoelectric point, the organic substance suspensions are more stable.

Having this in mind, the coagulation activities of extract of bean seeds were determined at pH values between pH = 6 and pH = 11. For these experiments, 3 mL of extracts per 1 L of water were used. The results are shown in the Graph 1.

As the results outlined in the Graph 1. show, the coagulation activity quickly increases at the pH values higher than pH 6,0. Similar results were obtained with the M. oleifera extracts [Okuda et al., 2001]. In further experiments pH values were adjusted to pH = 9.

In this experiment, the influence of agents for adjusting of the pH values of model water on the coagulation process was examined.

For these purposes, the solutions of NaOH or Ca(OH)2 were used, and the pH values were adjusted to pH 8 or to pH 9. The results are shown in the Graph 2.

It can be concluded that the pH value adjusting with the Ca(OH)2 at pH 9 gives slightly better results than the NaOH solution with respect to the coagulation activity. The reason is that the Ca(OH)2 solution brings into water the twovalence ions, so that the coagulation is improved owing to the formation of net structures with the coagulants, which improves the formation of floccules [Okuda et al., 2001]. Besides to that, Ca(OH)2 is less expensive than the NaOH which is additional advantage. But, considering the instability of the Ca(OH)2 suspension, in the rest of laboratory experiments, nevertheless, the NaOH solution was used for adjusting the pH values in the model water.

Investigation of the effects of extraction time and temperature of extraction on the coagulation activity of the bean seed extract

Investigation of the effects of mixing time on the coagulation activity of bean seed extract

CONCLUSION