Introduction
Conventional water treatment consist of unit processes include coagulation, flocculation, clarification, filtration and disinfection that goal is removal of turbidity, color and certain bacterial species. In filtration process flocculated substances that have not removed in pervious stage are sediment. Filter media is often silica sand that removal mechanism is complex and consists of mechanical filtration, sedimentation, adsorption, chemical and biological oxidation. The effective removal mechanism in filtration process is adsorption. Two basic factors for adsorption are van der walls force and mass attraction. Sometimes for quantifying and qualifying upgrading of filters, we use other media such as granular activated carbon and anthracite, that in these cases filters have dual media. Activated carbons (ACs) are effective adsorbents in water treatment, because of high removal capability toward soluble and particulate pollutants but slow adsorption kinetics and difficulty for regeneration limited extensive application. Todays, progresses in new technology result in production of other carbon structures such as carbon nano tubes (CNTs). CNTs with mesopore structures and diameter about 2 - 50 nanometer have high potential in organic materials and pathogens removal. Removal of organic matters with carbon nano tubes are in groups such as organic dyes, pharmaceutical, pesticides, phenol, aromatic amines and natural organic matters. Adsorption mechanisms of CNTs are driving including hydrophobic effect, π–π interaction, π–π electron-donor-acceptor (EDA) interaction, electrostatic interaction and hydrogen bonding. Although good potential in organic removal, using in slurry state is a problem that it is necessary to remove CNTs in final treatment. So, if we can capable to deposition these CNTs on media, we can solve this problems. One way to use is deposition of CNTs on silica sand. Although silica sand has adsorption capability, with deposition of CNTs it can remove soluble organic matters. Similar to this coating graphene oxide (GO) was deposited on silica gel that new adsorbent used for mercury and rodamin B removal in batch and continues experiments. One of the shortages in this research was inattention in stability of coating. In other research, pristine single wall carbon nanotube was coated on silica sphere and a method based on non-covalent bonding was represented. The result of this research shows that, bonding has a strong stability.
Methods
In order to deposition of CNTs on silica sand, physical and chemical processes were used. In physical process heat and in chemical process covalent bonding is effective factors for bonding. Materials that used in this research are carbon nano tube (purchased from nanosov), hydrochloric acid37% (Merck, Germany), nitric acid 65% (Merck, Germany), silica sand (gift from Tehran pars plant) and 3(triethoxysilyl) - prophylamin (Merck, Germany). Whereas heating may reinforce the bonding between carbon nano tube and silica sand, define temperatures was used to evaluate temperature effects.

For evaluating adsorption effects, 4 parameters (pH, adsorbent dose, adsorbate dose and contact time) was investigated. In this study initial pH at neural condition, initial adsorbent in 5-40 gr, initial adsorbate in 2- 12 mg /l and contact time in 10 – 60 minutes was adjusted. For preparation different concentration of TOC a 1000 mg/l stock solution of TOC was prepared and TOC analyzer (model -VCSH, Shimadzu, Japan) for determination of TOC was used.
For reversibility of humic acid from surface of new adsorbent adsorption/desorption studies was investigated. So in some series 20 g of engineered nano adsorbent was entered in 200 ml solution of humic acid with initial TOC of 10 mg/l and when adsorption process was equilibrated the adsorption capacity was measured. After that the new adsorbent was poured in 200 ml solution that pH was adjusted from 10-13 with 0.05 and 2 molar NaOH to attain optimum pH. Also desorption time for 10-50 minutes for evaluating optimum time was done. Finally desorption process at 25ºc, 120 rpm and for 5 cycles was repeated.
Results and discussion
Investigation method of chemical bonding
The method of chemical deposition is establishment of covalent bonding between amin groups of silica and hydroxyl groups of oxidized carbon nano tube. So first, silica sand is hydrolyzed to active OH groups. Then silica sand and amin prophyl is combined to produce NH2 groups and finally oxidized carbon nano tubes and amin groups of silica sand is heated to be resulted –NH groups that is a strong bonding.
Stability of MWCNTs coated on sand
To investigate stability of MWCNTs coated on sand, ultrasound method was used. Ultrasounds are mechanical waves with high power that disperse in adsorbent to detach CNTs from surface of silica sands. These waves have 40 khz dispersion power frequency. So 6 samples (firstly physical and chemical samples, physical and chemical samples was heated in oven in 100ºc and physical and chemical samples was heated in oven in 200ºc) was putted in the ultrasound bath for 30 min and after that samples was dried and it was seen that physical deposition has not stability and the MWCNTs coated on sand detached from silica sand, but chemical deposition has a strong stability and in samples that was putted it in 200ºc in oven, this stability is very permanent that demonstrate high temperatures has good effects on stability.
SEM of MWCNTs coated on sand
SEM was used to detect possible morphological changes in pristine and oxidized MWCNTs and also for watching surface of silica sand and MWCNTs coated on silica sand. According to SEM, oxidized MWCNTs with nitric acid, some bundle appear exfoliated and curled and a major alteration of the structural integrity of MWCNTs is observed and length of tube was shortened. On the other side, observation of CNTsand illustrate a non uniform coating of MWCNTs on silica sand that completely differ from surface of silica acid treatment.
Investigation potential of CNTsand in removal of organic matter
Although MWCNTs potential in removal of organic matter has been demonstrated in some previous research, in this study humic acid was used for index of organic matter to define capability of this adsorbent. So response surface methodology was used for experiment design and initial concentration of adsorbent, adsorbate and contact time was investigated and 20 experiments were defined. According to response from RSM- X/M (mg/g) – with increasing in contact time and decreasing in adsorbent dose the adsorption capacity was increased. Also with increasing in contact time and increasing in adsorbate dose, adsorption capacity is increased that be because of increasing in adsorbate collision with adsorbent. The equilibrium time is about 60 minutes and adsorption capacity at equilibrium time is reached to above 70 mg/g
Regeneration of adsorbent
To evaluating regeneration of adsorbent the adsorption/desorption process was investigated. Data obtained indicate at pH =10 the regeneration efficiency is about 26.3% and increases up 75.7% at pH=13. Also 30 minutes is the best time for regeneration time and after 5 cycles of regeneration, the adsorption capacity is about 40 mg/g.
Conclusion
Although silica sand is not applicable to removal of soluble organic matter, but by deposition of carbon nano tubes onto its surface, it can capable to remove this organics. Stability investigations indicate that physical coating has a week bonding between silica sand and carbon nano tubes. In chemical bonding that establish a covalent bonding between silica sand and carbon nano tubes, bonding has a strong stability. Scanning electron microscopy (SEM) on silica surface reveals a non uniform coating of carbon nano tubes on silica sand. Determination of TOC shows in sample with CNTsand , adsorption capacity is higher than to 70% and in sample with raw sand is less than 1%. Regeneration studies indicate that the optimum pH is 13 and optimum time for desorption is 30 minutes. Also after 5 cycles of regeneration the adsorption capacity is about 40 mg/g. These experiment show that the new adsorbent - CNTsand - has a good potential in organic matter removal and regeneration process is simple.
Keywords
Sand filter, silica, carbon nano tube, organic