Study On Removal Of Iron And Manganese From Groundwater By Contact Oxidation Process

China has abundant groundwater resources,and groundwater is a main water resource for the rural and urban mining enterprises in China.To some extent,the groundwater quality in Northeast is good,but it is commonly polluted by Fe and Mn. The superfluous iron and manganese minerals et al in the groundwater can cause a huge negative impact on industrial and agricultural production and people's lives.With the development of the society and the progress of science and technology,people become gradually aware of the hazardous of the superfluous iron and manganese.The superfluous minerals must be removed to ensure the industrial and agricultural production and people's lives is good.In general,there are four basic methods to remove Fe and Mn from groundwater:â‘ Aeration oxo-processâ‘¡Contact oxo-processâ‘¢Medicament oxo-processvâ‘£Biological oxo-process.In some small towns in Northeast,the consumption of groundwater is relatively small,and is not necessary to use complicated and huge equipment,and also is not economical.Because contact oxo-process have some virtues of simplicity equipments,relative small scale and the high efficiency of removing of Fe and Mn,it is very suitable for the local use.The mechanism of contact oxidation to remove Fe was catalytic oxidation,and the core of the catalytic reaction was Fe filter membrane-Fe(OH)₃·2H₂O,and it could adsorb Fe²⁺ in the water,and then Fe²⁺ oxidized rapidly to Fe³⁺,and the process was a spontaneous catalytic reaction.The mechanism of contact oxidation to remove Mn was that Mn²⁺ oxidized rapidly to MnO₂^- by the dissolved oxygen in the water,which occurred under the catalytic reaction of surface Mn activity membrane-MnO₂·xH₂O once the raw water enter the filter,which made the membrane update rapidly,and the process was a spontaneous catalytic reaction,too.All of above show that the formation of active filter membrane was the key to remove Fe and Mn,and the character of filter materials,the speed of flow and the intensity of backwashing were necessary factors in the formation of active filter membrane.Because the filter material contact directly with the groundwater,and also plays an important role in removing of Fe and Mn,the performance of the filter material in the formation of membrane is particularly prominent.The speed of Fe and Mn removal reaction will be fast onc the active filter membrane formed.Therefore,the time of forming the active filter membrane is the control-time of the whole process which removes Fe and Mn from groundwater.In the early 1960s,China has begun to remove Fe and Mn from groundwater using contact oxidation process.When using pyrolusite to be filter material in contact oxidation process,the mature filter material can oxidize Fe²⁺ to Fe³⁺ in only a few minutes,or even in a shorter time.Besides,quartz sand,shale,anthracite and river sand also have varying degrees of removing Fe and Mn.Among these materials,sand,shale have small specific weight and bulk density,lighter body,larger filter size,larger contact area of water,and they are cheap and easily obtained.So the first step of this experiment was to use the three kinds of media for dynamic experiments,by measuring the maturity time to choose the optimum filter.There are four types of modified shale, which were obtained under the conditions of 500℃,600℃,700℃and 800℃after three hours firing in muffle furnace,respectively.The first part of this experiment was to fill the organic column of 5cm diameter and 10cm height with 1-2mm diameter manganese sand,river sand and modified shale particles as stuffing.The influent concentration of Fe and Mn were 4.00—4.30mg/L and 1.00—1.10mg/L,respectively,and the flow rate of influent was 10ml/min.and the dynamic test begun to operate after 24-hour continuous flow influent.The backwashing test begun when the quality of effluent become worse,and the cycle of backwashing was 7 days,and the strength was 18-20L/m²Â·s.The experimental results showed that regardless of the kind of media.Fe was more easily removed,and Mn was relatively, difficult to be removed.The formation activity Fe membrane's time(the concentration of Fe was reduced to the national standard 0.3mg/L) of the six filter(manganese sand, river sand,and the 500℃,600℃,700℃,800℃-modified shale) were different,and the sequenee was:500℃-modified shale=600℃-modified shale(16d)＜sand=Mn=700℃ -modified shale(19d)＜800℃-modified shale(20d).The formation activity Mn membrane's time(the concentration of Mn was reduced to the national standard 0.1mg/L) of the six filter were different,and the sequence was:600℃-modified shale (23d)＜500℃-modified shale(24d)＜700℃-modified shale(26d)＜sand＜Mn=800℃-modified shale(28d).From the above data,we know that the 500℃and 600℃modified shale were the best,however,from the point view of economic and the situation of the local,shale need for grinding,screening and burning to be filter materials,which consume a large amount of energy.Although the treatment efficiency of river sand was worse than modified shale,it is easily access to local-in the Northeast of china,and sand only need screening for lower energy consumption;therefore,we chose river sand as filter material.Having chosen the sand as filter media,the second part of the experimental was to study the effect of the size of sand particle,water flow rate and mixing mature media on the maturity.Experiment results showed that the treatment efficiency with 1-2mm diameter particle was better than the diameter of 2-3mm and 0.5-1mm.and the reasons was the larger area of contact oxidation because of the size of surface of 1-2mm diameter greater than 2-3mm,and the clearance of 1-2mm particle was slightly larger than the generated oxide particles,the filter media was difficult to be plugged,the frequency of backwashing was low;The time of the sand become mature with 10mL/min influent flow rate was slightly shorter than 20mL/min influent,and the reason was it's easy to generate the oxide film with the low flow rate and generate a small impact on the oxide film;When mixed with mature media with 1:1 or 1:2,the maturity time of Fe reduced 8 days from 19 days,and the maturity time of Mn reduced 15 days from 27 days,which showed that it is feasible to mix proper amount mature filter media with the new filter media,and it could shorten the time of the maturity of the new media about 1～2 times.If the proportion of mature media mixed with new filter media was higher,the effect was much better.The third part of the experiment was the experimental enlarge.Firstly,filling the organic column of 5cm diameter and 80cm height with river sand.,and there was a sample point every 10cm.It was operating until maturity at a flow rate of 20mL/min. and then through two times boost speed to make penetrate the media(that is,the concentration of Mn could not in line with the national standard any more at the final sampling point).The experimental results showed that the concentration of Mn was still changing below the height of 80cm.so it is necessary for more experimental enlarge.The experimental results showed that the optimum filter media was river sand while using contact oxidation to removal Fe and Mn.and it was feasible in terms of both remove efficiency and economic considerations,and the experimental results could considered be some basic operating parameters for water treatment factory's actual operating test.