Colour removal from pulp and paper mill effluent using lignite as an adsorbent
Treatments that are usually employed by pulp and paper industries comprise sedimentation, aeration, activated sludge processing and anaerobic treatment. These methods have proven to be effective for removing suspended solids and for significantly decreasing COD and BOD. However, the resultant water still has colour, which is generally unacceptable to the community. This leads to the need for colour removal as a secondary stage in wastewater treatment. Some treatments that have been reported to deal with colour removal are photocatalysis, oxidation, electrocoagulation, biological treatments and adsorption. Among these methods, adsorption is still considered to be one of the simplest and economical methods. Activated carbon is probably the most prominent adsorbent used in wastewater treatment. Unfortunately, the high costs associated with its activation, regeneration and maintenance have been significant drawbacks to its use. Lignite, a low rank coal, is a cheap and readily available material that has similar characteristics to activated carbon. Here we report on the use of lignite as an adsorbent for colour removal from treated pulp and paper mill effluent. Two types of experiments were carried out. In the batch experiments, various masses of lignite were added to the wastewater. While in the filter bed experiments, the wastewater was passed through a fixed bed of lignite and the effluents leaving the bed were collected and examined. The results demonstrated the ability of raw lignite to remove some colour, TOC and total phosphorus from the effluents. It was found that the adjustment of the pH of the wastewater was crucial due to some colour leaching from lignite. These preliminary results suggest the potential of lignite, as a cheap and readily available adsorbent, to be utilized in wastewater treatment.
pulp and paper effluent, colour removal, lignite, adsorption.
Compounds that are present in pulp and paper wastewaters include wood polymer, natural inorganic fillers, process chemicals and reaction products in the form of suspended solids, colloidal and dissolved matter. The toxicity of the wastewater is typically due to chlorinated lignin-based compounds such as phenols, catechols, guaiacols and aromatic hydrocarbons. There are also tannins, resin and acids from woods. Thus, there is blend of hundreds of different organic as well as inorganic compounds, including neutral, acidic and basic, oxidisable and non-oxidisable, stable and unstable, toxic and non toxic components, where some species can undergo chemical reactions with others, producing a highly complicated mixture.  The complexity of wastewater from pulp and paper mills makes it an interesting yet very challenging subject to be treated and analysed. Considering the large scale of this industry and the international production of pulp and paper, any improvement on the water effluent quality will significantly benefit the environment.
The primary treatment of pulp and paper mill effluent that is usually employed at plants is clarification. Secondary treatment may be a physicochemical process, biological process or combination of these two. There may also a tertiary treatment, to deal with colour removal. 
Approaches that have been reported to deal with colour removal include physicochemical treatment, biological treatment and integrated treatments which combine two or more processes. Stephenson and Duff observed the removal of 90% of colour from a mechanical pulping effluent when using iron and alum as coagulants . Ugurlu and co-workers used electrocoagulation to specifically remove phenol and lignin, compounds that are considered to be responsible for colour development in the wastewater . Zhang and Chuang studied the adsorption of acidic bleach plant effluent on activated carbon and on a polymer resin and discovered the resin to be more effective for colour removal from pulp mill effluents . Tarlan and co-workers reported the removal of 84% of the colour from a pulp and paper industry wastewater using algae treatment . Rodrigues and co-workers reported the removal of 100% of the chromophores absorbing at 500 nm by a coagulation-flocculation technique followed by heterogeneous photocatalysis using an UV/TiO2/H2O2 system . Ghoreishi and Haghighi reported the use of hydrogenation in biological batch reactors to remove chromophores in pulp and paper effluent .
On the other hand, amongst other tertiary processes, adsorption has been a very popular technique. This technique is preferred when dealing with organic compounds removal at lower concentrations. Activated carbon is one example of the most used adsorbents. Unfortunately, It is no longer considered to be the most economical method due to its high activation, regeneration and maintenance costs.
Lignite deposits in the Latrobe Valley in Australia are exceptionally large. Unfortunately, the use of lignite as fuel is rather inefficient due to its high water content. The alternative use of lignite has always been an attraction for industry and government, as it could make use of a cheap and abundant natural resource .
Butler and co-workers successfully prepared an adsorbent bed of lignite and used it to treat wastewater that resulted from a non-evaporative brown coal dewatering technique called Mechanical Thermal Expression (MTE). This method achieved 60% reduction in COD and also removed about 25% of the total cations. Although the research was not specifically aimed at colour reduction, it revealed the potential of lignite as an adsorbent and also as an alternative for costly activated carbon. 
In this paper, lignite has been used as an adsorbent in pulp and paper mill wastewater treatment. Two types of experiments, namely the batch experiment and filter bed experiment have been conducted. The properties of water (TOC, colour and total phosphorus) before and after treatment were compared, and the lignite selectivity to these contaminants was discussed.
2.1 Sample water
The wastewater used in this study was the treated effluent from a pulp and paper mill. It was a clarifier effluent that was then subjected to aeration in a sedimentation pool. Some parameter that has been measured were TOC, pH, conductivity, total phosphorus and colour, as can be seen in Table 1.
Table I Properties of the treated wastewater
Total Phosphorus (mg/L)
2. 2 Batch experiment
Batch experiments were carried out by shaking desired amounts of raw lignite with 100 mL of wastewater. The solution and the adsorbent were stirred in an Erlenmeyer flask for 10 minutes using a magnetic stirrer. To separate the solid phase, the solution was kept for approximately 1 day.
2.3 Filter bed experiment
The setup used in this experiment was based on Butler et.al work, as illustrated in 1. Raw LY brown coal is inserted into a glass column as slurry with pH-adjusted water. The column was then shaken until the coal bed reaches a certain height. The coal bed was then washed with pH-adjusted water, the wash water being provided via a 1L dropping flask (used as header tank) to provide a desired flow rate. Samples of the effluent are then collected at specified volume intervals.
2.4 water analysis
Initial and resultant effluents were measured for their colour, total organic carbon, total phosphorus, pH and conductivity. The colour measurement was carried out using HACH DR5000 Spectrophotometer (Method number 8025). Total Organic Carbon (TOC) was measured using Shimadzu TOC-V Analyzer. Total Phosphorus was measured using Lachat QuikChem® 8500 Automated Ion Analyser via a colourimetric FIA method.
3. RESULT AND DISCUSSION
3.1 Batch experiment
Before running adsorption experiments, the lignite leaching as a function of solution pH was investigated. The result is illustrated in 2.
From 2, it can be seen that the increase in solution pH led to an increase in lignite colour leaching. The pH solution ranging from 4.2-4.3 was then chosen for the entire adsorption study. This pH is chosen because relatively small amount of colour leached at this point and small adjustment was needed from the initial solution. Adjustment to a lower pH, for example to 2.5, will be uneconomical.
The relationship between the mass of lignite to colour and phosphorous removal was investigated; the result can be seen in The 2.
The result obtained in batch experiment demonstrated the lignite ability to act as an adsorbent for colour and phosphorus removal from pulp and paper mill effluent. It can be concluded that addition of 5 g of lignite to 100 mL of wastewater was able to remove more than 70% of colour and over 50% of total phosphorus.
3.2 Filter bed experiment
Prior to adsorption, the filter bed prepared for the experiment was washed using deionised water with adjusted pH. The pH of the washing water was 4.3 and the conductivity was 65 µS. This washing stage is crucial to ensure that the lignite will not leach any material during adsorption that may interfere with effluent analysis.
The washing process was monitored through conductivity measurement. This method offers a convenient way to monitor the lignite mineral leaching that was indicated by the increase in the conductivity of the resulting effluent. The result is illustrated in 3.
It can be concluded that the lignite did leach some ions during washing as indicated by the higher conductivity value for the first 200 mL of washing effluent. However, the conductivity of the following effluents was at the same value of initial wash water. This result showed that around 36 BV of washing water is needed to wash lignite bed prior to adsorption.
Some ion/salts released by the lignite during washing were probably originated from salts deposit in the interstitial pores of lignite. The high water content of lignite, ranging from 50%-70%, was one reason for its high dissolved salts deposit. Ions were also released as a result of ion exchange due to the high functional group content of the lignite and the acidic nature of the wash water.
Aside from conductivity measurement, the resulting washing effluents were also subjected to pH and total phosphorus measurements. The changing of effluents pH during washing is illustrated in 4.
It can be seen from 4 that the filter bed of lignite established the new equilibrium pH. The initial pH of wash water was 4.3 and equilibrium pH of the bed was 3.9. This result demonstrates the buffer capacity of the bed of lignite.
In term of phosphorus leaching of lignite, it was found that the lignite only leached a small amount of phosphorus. The total phosphorus of washing effluent for the first 30 mL, 60 mL and 90 mL of washing effluents were 0.07 mg/L, 0.04 mg/L, and 0.03 mg/L, respectively.
After ensuring that the filter bed of lignite did not experience further leaching, the pH-adjusted wastewater was then passed through the bed. The lignite selectivity was investigated, particularly in term of colour, organic and phosphorus removal. The result is shown in 5.
Similar to batch experiments, the result once again shows that the lignite has the ability to remove some colour, organic carbon and total phosphorus from the wastewater. It is arguable that the lignite is more selective to colour removal than to organic carbon or phosphorus. However, the removal of 40% to 50% of organic and total phosphorus can be maintained for up to 200 mL of resulting effluent. In this experiment the saturation of colour adsorption was not achieved. Much more wastewater is probably needed to achieve saturation.
Another experiment to investigate the effect of filter bed volume and flow rate on adsorption had also been carried out. The result is illustrated in 6.
The adsorption was analysed in terms of colour removal from the wastewater effluent. It can be concluded that the increase in flow rate led to the decrease in colour removal. This is understandable since the lignite requires longer contact time to be able to optimize the adsorption. The bed volume also had positive correlation to the adsorption. The increase in the bed volume of lignite also led to better adsorption.
To quantify the lignite capacity, the result for TOC removal was analysed further. The amount of TOC removed during adsorption and the lignite capacity was quantified. The relation between TOC adsorbed and the effluent volume is illustrated in 7.
It can be conclude that 10 g of lignite was ‘only' capable of removing 16 mg of TOC from the wastewater. In other way, the lignite capacity for TOC removal was 1.6 mg/g.
Lignite has also shown a promising ability to remove colour, organic and total phosphorous from the wastewater by adsorption. One disadvantage that was found is Lignite's tendency to leach small amount of colour when introduced to a water solution. However, this process was found to be a pH-dependent process and can be avoided by adjusting the pH to around 4. In the case of filter bed experiments, prewashing the bed is crucial to eliminate some material leached by the lignite that may interfere with the analysis.
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