The potential application of photocatalytic processes in the processing of wastewater in the leather industry: A Review

The leather industry is an industry that is not environmentally friendly. This is due to the large amount of solid and liquid waste produced. In every processing of 1 ton of raw skin into leather it takes about 50-150 liters of water and about 300 kg of chemicals. The chemicals commonly used in the production process are chromium, sulfate, sodium sulfate, lime, ammonium sulfate, sodium chloride, sulfuric acid, formaldehyde, pigments, dyes, and antifungal agents. These chemicals make the intensity of the poison produced per unit of output high. In Indonesia, chromium is a tanning agent that is widely used because it is cheap, the tanning process is fast, and it produces stable leather. Disposing of leather tannery wastewater directly without prior treatment can cause serious environmental problems due to the high content of COD, BOD, chromium and dyes. Leather industry wastewater contains around 500-1000 ppm chromium (VI). Several efforts to treat Cr(VI) waste that have been carried out, such as chemical reduction, ion exchange, adsorption with coal or activated carbon and reduction with the help of bacteria have weaknesses, namely the need for very high energy and/or very large amounts of chemicals. This weakness makes the photocatalytic method more prospective and superior for application. This paper was created to briefly review the environmental risks posed by the leather tanning industry, especially Cr(VI) waste, as well as the potential for the application of a photocatalytic process to remove these pollutant parameters.


Introduction
The leather industry is an industry that is not environmentally friendly.This is due to the large amount of solid and liquid waste produced.In every processing of 1 ton of raw skin into leather it takes about 50-150 liters of water and about 300 kg of chemicals.The chemicals commonly used in the production process are chromium, sulfate, sodium sulfate, lime, ammonium sulfate, sodium chloride, sulfuric acid, formaldehyde, pigments, dyes, and antifungal agents.These chemicals make the intensity of the poison produced per unit of output high.In Indonesia, chromium is a tanning agent that is widely used because it is cheap, the tanning process is fast, and it produces stable leather.Disposing of leather tannery wastewater directly without prior treatment can cause serious environmental problems due to the high content of COD, BOD, chromium and dyes.The leather industry wastewater contains around 500-1000 ppm chromium (VI) [1].
Chromium (VI) can be toxic to animals and plants.Cr(VI) waste is becoming popular due to its carcinogenic properties.Chromium occurs in nature in 2 oxide forms, namely Cr(III) and Cr(VI) oxides.Interestingly, only Cr(VI) is carcinogenic while Cr(III) is not.The toxicity level of Cr(III) is only about 1253 (2023) 012025 IOP Publishing doi:10.1088/1755-1315/1253/1/012025 2 1/100 that of Cr(VI).Even from further research, it turns out that Cr(III) is a type of nutrient needed by the human body with levels of around 50-200 μg/day.Cr(VI) easily dissolves in water and forms divalent oxyanions, namely cromate (CrO4 2-) and dichromate (Cr2O7 2-), while trivalent Chromium/Cr(III) is easily precipitated or absorbed by organic and inorganic compounds at neutral or alkaline pH [2].Several Cr(VI) waste treatment efforts that have been carried out such as chemical reduction, ion exchange, adsorption with coal or activated carbon and reduction with the help of bacteria have weaknesses, namely the need for very high energy and/or very large amounts of chemicals.This weakness makes the photocatalytic method more prospective and superior for application [2,3].
This paper was created with the aim of briefly reviewing the environmental risks posed by the leather tanning industry, especially Cr(VI) waste, as well as the potential for applying a photocatalytic process to remove these pollutant parameters.

Characteristic of leather tanning industry wastewater
The liquid waste from the leather tanning industry is generally cloudy in color and has an unpleasant odor [4], because it generally contains the remains of meat and blood, lime pulp, fine hairs, dissolved protein, residual salts, acids, residual paint and chromium tanning substances.Chromium (Cr) is a metal with oxidation values ranging from 2+ to 6+ Cr, but usually exists as trivalent Chromium Cr(III) and hexavalent Chromium Cr(VI) (Cheung et al. 2007).Chromium in the form of Cr(VI) has the highest level of toxicity, around 10 to 100 times that of Cr(III) [5].The United States Environmental Protection Agency has classified Cr6+ as one of 17 chemicals toxic to humans.It is considered as one of the 20 contaminants that need to be treated before being discharged into the environment.Table 1 shows the chemical content contained in the leather tanning industry wastewater. 3. Risk arising from the presence of chromium in the leather tanning process Toxic effects will arise, if inhaling contaminated workplace air, such as in leather tanning.The toxic effects of chromium can be damaging and irritating to the nose, lungs, stomach and intestines.High long-term effects of chromium cause damage to the nose and lungs.Consuming very large amounts of chromium-based foods can cause stomach upset, ulcers, seizures, kidney, liver damage, and even death.The effect of chromium on health is that it can experience respiratory problems and also interfere with the digestive system.Chromium(VI) is known to cause various health effects.When chromium is an ingredient in leather products, it can cause allergic reactions, such as skin rashes.After inhaling Chromium(VI) can cause nasal irritation and nosebleeds.Acute poisoning will cause several conditions, namely: • Inhalation / inhalation: If chromium dust or vapor is inhaled at high concentrations it may cause irritation.• In contact with skin : Direct contact with dust or chromium powder may cause skin irritation.
• In case of eye contact: Direct contact with chromium dust or powder may cause eye irritation.
• If swallowed: Chromium metal is very difficult to absorb through the digestive tract.Absorption of sufficient quantities of some chromium compounds can cause dizziness, extreme thirst, abdominal pain, vomiting, shock, oliguria or anuria and uremia which may be fatal.
While chronic poisoning will cause the following conditions: • By inhalation / inhalation: Repeated long-term exposure to some chromium compounds has been reported to cause ulceration and perforation of the nasal septum, irritation of the throat and lower respiratory tract, digestive tract disorders, but this is rare occur, blood disorders, lung sensitization and pneumoconiosis or pulmonary fibrosis and effects on the liver are rare.In fact, this effect has never been reported due to metal exposure.• On skin contact: Repeated long-term exposure to some chromium compounds has been reported to cause various types of dermatitis, including sensitizing eczema "Chrome holes" and skin and kidney damage.In fact, this effect has never been reported due to metal exposure.• In contact with eyes: Repeated long-term exposure to some chromium compounds can cause inflammation of the lining of the eye (conjunctivitis) and lacrimation.In fact, this effect has never been reported due to metal exposure.

Photocatalytic
Photocatalytic is the acceleration of reactions induced by light due to the presence of a light catalyst or what is commonly called a photocatalyst [7].Photocatalytic is another name for "Advanced Oxidation Processes" (POLs) which are based on the production of highly reactive species, namely hydroxy radicals (OH•) which can oxidize various kinds of organic substances quickly and not selectively [8].In heterogeneous photocatalysis, the light catalyst is in the form of a solid.semi-conductor.Photocatalytic can be carried out in a gas or liquid state.The process of photocatalytic oxidation of organic compounds in water can be explained in the following mechanism: • Diffusion of reactants from the liquid through the barrier layer to the catalyst interface in the solution.
• Inter/intraparticle diffusion of the reactants towards the surface of the active side of the catalyst.
• Adsorption of at least one type of reactant.
• The reaction is in the adsorbed phase.
• Release of product from the interface to the liquid The photoinduction reaction in step four is activated by means of photon adsorption with sufficient energy [7].Here is the mechanism of the photocatalytic oxidation reaction with the help of UV light [9]: Factors that can affect photocatalytic include [10][11][12][13]: reactor design, light wavelength, light intensity, photocatalyst loading, initial concentration of reactants, temperature, pH, oxygen content, and the presence of ions.Photoreactors can be divided into two main parts: (1) fixed bed photoreactors and (2) batch slurry photoreactors.Both of these reactors have advantages and disadvantages of each.This reactor is characterized by the ratio between surface area and volume and its ineffectiveness as seen from its ability to absorb and scatter light in the reaction medium [14].Light wavelength; the most commonly used photocatalyst such as TiO2 absorbs irradiation below the visible light range so that at least the light needed is UV light which can be obtained from artificial lamps or solar radiation.
Light intensity; the effect exerted by light intensity on the kinetics of photocatalytic reactions can be characterized as follows [15]: (1) at low intensity (0-20mW/cm 2 ), the reaction rate increases linearly with increasing light intensity (first order); (2) at an intermediate intensity of approximately 25 mW/cm 2 , the reaction rate depends on the root of the light intensity (half order); and (3) at high intensity the reaction rate does not depend on the light intensity.The most commonly used light source is an artificial UV light lamp which emits UV-A light with a maximum wavelength of between 355-365 nm or UV-C in the form of an anti-germicidal lamp with a maximum wavelength of around 254 nm.Artificial UV light lamps are often used because they can produce a larger photon flux.
Photocatalyst loading; in general, increasing the catalyst load can increase the surface area of the catalyst available for adsorption and degradation.Conversely, increasing the concentration of the photocatalyst increases the opacity of the solution which results in a reduction in the penetration of photon flux in the reactor.The initial rate of the photoreaction is directly proportional to the mass of the catalyst present in the solution.However, when the catalyst charge exceeds a certain limit, the reaction rate becomes independent of the amount of catalyst and the reaction rate will be constant or even decrease with increasing catalyst concentration.
Initial concentration of reactants; in general, the rate of degradation of organic pollutants increases as the initial concentration of the pollutant increases to a certain level and a subsequent increase in concentration leads to a reduction in the rate of degradation [12].Temperature; photocatalytic systems do not require heating and generally photodecomposition is operated at the recommended room temperature between 20-80°C [10,12].At very low temperatures i.e. below 0°C the activity decreases and the rate limiting step becomes desorption of the end product from the catalyst surface.Conversely, if the temperature increases above 80°C and approaches the boiling point of water, exothermic adsorption of pollutants becomes undesirable and becomes a rate limiting step so that activity decreases [10,12].At 20-60°C, increasing the reaction temperature can increase the rate of photodecomposition.pH value; the effect of pH on the photocatalytic degradation of organic compounds in water is related mainly to (I) the surface ionization state of TiO2, (II) the valence position and conduction band of the photocatalyst, (III) the agglomeration of TiO2 particles and (IV) the formation of hydroxyl radicals [12].The effects caused by pH are very complicated and complex so that the optimal pH value required for each application needs to be determined on the basis of initial investigations.Oxygen Content; oxygen plays an important role as an electron collector in the photodegradation process.It was found that the presence of oxygen might inhibit or increase the rate of photodegradation depending on the pollutant degradation mechanism [10].Presence of ions; natural inorganic anions in waters, such as Cl -, NO3 -, SO4 2-, CO3 2-and HCO3 -, act as holes (h+) and hydroxy radical collectors [12].As a result, radical inorganic anions, eg NO3•, CO3•-, etc., are formed.Although the reactivity of these radicals can be considered, they are not as reactive as h+ and OH•.Therefore, a decrease in photodegradation efficiency in the presence of inorganic ions is usually observed, which is mainly due to the adsorption of these ions on the TiO2 surface [12].

Implementation of cleaner production
Cleaner production is a preventive and integrated environmental management strategy that needs to be implemented continuously in the production process so as to reduce negative risks to humans and the environment.Cleaner production in the production process means increasing the efficiency and effectiveness of the use of raw materials, energy and other resources, and replacing or reducing the amount and toxicity of all emissions and waste before leaving the process.Prevention, reduction and elimination of waste or polluting materials at the source is a key element in cleaner production.Activities that constitute net production are (1) saving the use of washing / rinsing water; (2) reducing the use of chemicals, for example tanning using chromium salt with a sufficient solution content of 8% does not need to use 12%; (3) modification of the process, such as in the liming process using a drum with the amount of materials used can be reduced (water, lime, sulfide) or by separating the liquid in the process of removing feathers and liming; and (4) use of appropriate technology and equipment.

Recycling chrom
Recycling chrom is done by separating the chrom first so that it can be used again.Chrom can be separated from the waste liquid by filtering which is then recycled by means of: waste water from chrom tanning and washing water (as much as 2 x 100% water) which is free of solids is given a solution of magnesium hydroxide, and precipitated for about 10 hours, then the liquid is transferred to another tub (with a suction pipe, but don't let the sediment get sucked in).If the liquid is completely free of sediment, it will contain less than 2 ppm of chromium so that it can be immediately disposed of or used for recycling.The precipitate that occurs is then added to the appropriate sulfuric acid, the precipitate will dissolve in about 15 minutes and will give a chromium solution of 50 grams of chromium oxide/liter.The next recycling process still requires the addition of approximately 30% chromium.

Reduction of Cr (VI) using TiO2 Photocatalyst
Cr(VI) reduction was carried out using a TiO2 powder catalyst and a titania-oxide film.Some of the variables studied were the effect of adding EDTA, the pH of the solution, the initial concentration of Cr(VI), and the effect of adding activated carbon on photocatalytic activity in reducing Cr(VI).Photoreduction of Cr(VI) waste was also carried out using a film catalyst using a circulating photoreactor.The photocatalytic activity was tested by measuring the decrease in Cr(VI) concentration with respect to irradiation time.The batch photoreactor used is made of Pyrex material, in the form of a stirred tank for powder catalysts and a circulation reactor for film catalysts.The source of photon energy is obtained from a Mercury Lamp (High Pressure Mercury Lamp) with a power of 175 watts from the General Electric brand [2].
The standard operating conditions used in the test were [2]: the reaction was carried out at pH 2, the addition of EDTA as a hole scavenger was carried out with the ratio [EDTA] : [Cr(VI)] = 5 : 2. The catalyst concentration used in the activity test was 5 g/l .Parameters tested included Cr(VI) reduction blank test, various types and forms of catalysts, effect of adding EDTA, effect of solution pH, variation of initial Cr(VI) concentration, and effect of adding activated carbon.The Cr(VI) reduction activity test was carried out using four types of catalysts from the preparations and two types of commercial catalysts (TiO2 Degussa P-25 and TiO2 Merck) as comparisons.The results of the catalyst variation test showed that TiO2 is the most active catalyst among other catalysts.If related to the BET and XRD characterization results, these results are logical because TiO2 has the largest surface area among other catalysts and has a crystal structure of 100% anatase.The addition of dopant catalysts to 2 different types of catalysts, namely Merck's TiO2 and TiO2 (prepared with TiCl4 as the starting material), actually actually decreased the activity of the catalyst.The addition of CuO dopant to Merck's TiO2 reduced conversion by 6.85%, while for the addition of Cu dopant to TiO2, the activity decreased by 6.1% and for CuO dopant, decreased by 22.45%.From these results it can be said that the addition of 1% dopant to Merck's TiO2 and prepared TiO2 had a negative effect on the activity of the catalyst in reducing Cr(VI) to Cr(III).This means that the amount of dopant loading added is not correct so that the dopant cannot function properly as an electron trap which should be able to inhibit the rate of electron-hole recombination.Cu loading of 1% added is suspected to be excessive, so that it could result in scoping/covering of the TiO2 surface which is where the Cr(VI) reduction reaction takes place.
The effect of the addition of EDTA shows that the addition of EDTA can increase the Cr(VI) reduction conversion by 11.7%.This increase is due to the fact that EDTA can function as a hole scavenger, thereby reducing the rate of electron-hole recombination.By decreasing the electron-hole recombination rate, it is hoped that more electrons will reach the surface of the catalyst so that more Cr(VI) is reduced.These results are in accordance with research conducted by [3].The effect of pH variations on the Cr(VI) reduction process shows that the lower the pH of the solution (the more acidic it is), the higher the Cr(VI) reduction rate.For a reaction time of 5 hours, at pH 10 Cr(VI) was reduced by 47.67%, at pH 7 it was reduced by 57.23% and at pH 2 it was reduced by 73.97%.This phenomenon can occur because the lower the pH, the concentration of H+ ions will increase.From the equation for the Cr(VI) reduction reaction, it is known that the overall reduction reaction of Cr(VI) requires or consumes protons (H+) so that with more/available protons, the reaction equilibrium will shift to the right and the rate of the Cr(VI) reduction reaction will increase [2].
The greater the initial concentration of Cr(VI) means that the number of molecules in the solution is also increasing and the ability of the catalyst to adsorb and reduce it is smaller.At an initial concentration of Cr(VI) 40 mg/L, the catalyst was able to reduce Cr(VI) up to 73.97%, at a concentration of Cr(VI) 60 mg/L, Cr(VI) was reduced by 61.5% and at a concentration of 80 mg/L, Cr(VI) was reduced by 47.27%.This shows that the reduction process of Cr(VI) at low concentrations has a faster rate [2].Testing the effect of adding activated carbon was carried out with the ratio [activated carbon]:[Cr(VI)] = 5:1, using a TiO2 catalyst (5 gr/lt) [2].It is known that the addition of activated carbon can increase the reduction conversion by 6.75%.The increase in Cr(VI) conversion is due to the synergy effect of the combination of catalyst and activated carbon.The synergy effect can occur because activated carbon can adsorb Cr(VI) molecules followed by mass transfer of Cr(VI) from activated carbon to the surface of the catalyst.
The activity of film catalyst with circulation reactor is still quite low.The low activity is more due to the ineffective circulating photoreactor system, so that the contact between the catalyst-reactant-UV is not optimal.It can also be evaluated that the reaction order = 1.46 and the reaction rate constant (k) = 0.115 [mol0.46/L0.46.hour], so the reaction rate equation is: -r [mol/L.hour]=0.115*CA 1.46  (10 where CA is the waste concentration of Cr(VI) in units of mol/L.

Conclusion
Efforts to reduce Chromium (VI) in leather industry wastewater can be done by: applying cleaner production, recycling and by photocatalytic processes.Based on literature review pure TiO2 catalyst prepared from TiCl4 has the highest activity in reducing Cr(VI) waste to Cr(III), with a conversion of about 80%.This is due to the high surface area and the anatase crystals formed.The addition of EDTA as a hole scavenger can increase the photocatalytic activity in reducing Cr(VI) by about 10%.The rate of reduction of Cr(VI) is faster at low pH and at a low initial concentration of Cr(VI).The addition of activated carbon as an adsorbent can increase the photocatalytic activity by about 5%.

Table 1 .
Chemical content of leather tanning industry liquid waste.