JP4008723B2 - Hexavalent chromium treatment method - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、廃水、焼却灰、汚泥及び土壌等の対象物または固化材として使用するセメント中に含まれる６価クロムを電気還元によって無毒化し処理する方法に関する。
【０００２】
【従来の技術】
廃水、焼却灰、汚泥及び土壌等に含まれる重金属及び化学物質等の環境中への溶出あるいは各種汚泥ケーキの再泥化を防止する技術として、セメントで固めて安定化させる方法がある。一方、重金属の内、６価クロムは皮膚潰瘍、肝障害、味覚及び嗅覚障害を起こす危険性を持ち、水質汚濁防止法で有害物質に指定されている。この6価クロムは、微量ながらセメントにも含まれ、セメントを添加し浚渫土等から改良土を製造する際には改良土からの６価クロムの溶出試験が義務付けられている。一般に６価クロムを無毒化する方法としては、還元剤あるいは電気還元で６価クロムを３価クロムに変換する工程、水酸化ナトリウム等で水酸化クロムの沈殿物にする工程、脱水処理し沈殿物を脱水ケーキとして排出する工程、更に安定型処分場に持ち込むためにセメント等で固める工程を経て安定化するものがある。
【０００３】
【発明が解決しようとする課題】
前記セメントで固めて安定化させる方法では、セメント中に元々含まれる６価クロムに対しては何ら処理がなされてないため、完全に無毒化し安定化されているとは言い難い。特に、廃水、焼却灰、汚泥及び土壌等の対象物中に含まれる６価クロムを対象とする無毒化方法においては、還元剤あるいは電気還元で３価クロムに変換する工程以降に排出される脱水ケーキをセメントで固めるという手順では、３価クロムとして無毒化した脱水ケーキにセメント由来の６価クロムを再度持ち込むことになる。更に、還元工程後の沈殿槽内でセメントを添加撹拌した後に水酸化クロムをセメントと共に脱水機で脱水処理しケーキとして排出した場合には、セメントに含まれる６価クロムがろ過水側へ溶出するという問題も持っている。
本発明は、これらの問題を解決するためになされたもので、対象物中の６価クロムを無毒化するだけでなく、セメント由来の６価クロムをも無毒化処理し、簡便に硬質状態の固形物中に封じ込める方法を提供することにある。
【０００４】
【課題を解決するための手段】
上記事情に鑑み鋭意検討した結果、対象物は、硬化能力があるセメントを加え懸濁液の状態で、６価クロムを３価クロムに電気還元することができ、更にセメントに起因するアルカリ条件下で３価クロムが直ちに水酸化クロムの不溶性化合物に変化することでセメントに巻き込んだ状態で脱水除去できることを見出し本発明を完成するに至った。
即ち、本発明は、廃水、焼却灰、汚泥及び土壌等の対象物または固化材として使用するセメント中に含まれる６価クロムを処理する方法で、前記対象物に硬化能力のあるセメントを加え懸濁液をつくる混合工程と、該混合工程から得られる懸濁液中に溶解する６価クロムを陽極と陰極とから構成される少なくとも一対の電極を備える電気還元槽にて３価クロムに電気還元する無毒化工程と、該無毒化工程から得られる３価クロムを水酸化クロムの形態でセメントと共に脱水処理し脱水ケーキとして処理する排出工程からなる６価クロムの処理方法である。
また、前記懸濁液が水素イオン濃度をｐH９以上になされていることを特徴とする。
更に、前記懸濁液が生コンクリートの混練設備や残存生コンクリートの処理工程から排出される硬化能力のあるセメントを含むスラッジ水を利用し調製されることを特徴とする。
【０００５】
【発明の実施の形態】
本発明は、６価クロムの電気還元による無毒化処理を硬化能力があるセメントを加え懸濁液の状態で実施することを基本としている。このセメントは、硬化反応により安定な固形物にする作用だけでなく、還元反応で生じた３価クロムを不溶性化合物の水酸化クロムにするためのアルカリ分の供給、更には効率的に電気還元を実施するための電解質の供給を行う作用を持ち、本発明はこれらの作用を積極且つ効果的に活用するものである。
本発明に係る６価クロムを含む対象物とは、メッキ廃水や電解加工廃水等の溶液状態でも、焼却灰や土壌等の固形物状態でも良い。また、生コンクリートの混練設備や残存生コンクリートの処理工程から排出されるセメントを含むスラッジ水等、本発明において硬化剤として用いるところのセメント由来の６価クロムを含んだ廃水も対象物となる。
【０００６】
本発明の混合工程は、対象物中に含まれる６価クロムを電気還元する前に、予め前記対象物をセメントを加え懸濁液にする。この際、セメントの添加量は、懸濁液の水素イオン濃度がｐH９以上のアルカリ性になるように調製する。また、対象物とセメントは別途設置する混合撹拌槽で懸濁液としても、あるいは電気還元を行う槽内で混合し懸濁液とすることもできる。更に、生コンクリートの混練設備や残存した生コンクリートの処理工程から排出される硬化能力のあるセメントを含むスラッジ水を利用する場合は、該スラッジ水に対象物を添加撹拌することで容易にセメントを加える懸濁液を調製することができる。
本発明に係る６価クロムの無毒化工程では、セメント及び対象物中の他の夾雑物と一緒に６価クロムを処理するが、セメントに自硬性があることから、電気還元槽の内壁、電極等への固着防止のため、懸濁液を淀みなく流動させる。流動させる手段としては、ポンプで循環させても撹拌羽で撹拌してもよく、これらを組み合わせて使用することもできる。処理後は電気還元槽の内壁、電極の付着物を取除くための手段を講じる。付着物を取除くための手段としては散水装置による洗浄あるいは清水注水による浸漬等をあげることができる。また、電気還元では電極に析出物が生じ電気抵抗が増加し易いことから随時極性反転の制御を行い、電極を洗浄することが好ましい。
【０００７】
使用する電極は、材質、形状及び大きさの制限はなく、作用効果、コスト及び使用条件等を考慮し選定される。電極の材質としては、白金、アルミニウム、鉄、ステンレス、チタン、マグネシウム合金、亜鉛、銅等の金属系材料及び炭素棒等があげられ、陽極と陰極は同じ材質を一対の電極として使用しても、異なる材質を組合せ一対の電極として使用してもよい。特に、鉄は入手し易いだけでなく、陽極に使用した場合は、還元作用の強い２価の鉄イオンを生じ、６価クロムの還元を補足する機能が高いため適した材質の一つである。また、電気還元する際の電気量は、対象物中の６価クロム濃度及び処理時間等を基に決定すればよく、通常、ファラデーの法則から求められる理論値よりも大きい値を使用する。
本発明の無毒化工程では６価クロムを３価クロムに還元し、排水処理工程では、3価クロムを水酸化クロムの形態でセメントと共に脱水処理し脱水ケーキとして処理する。脱水を行う機器としては、フィルタープレス機、遠心脱水機、ベルトプレス機等が使用できる。脱水ケーキは一定期間養生することで硬化させ、最終的に水酸化クロムを安定な固形物中に封じ込める。
３価クロムから生じた水酸化クロムは、セメントを含む懸濁液中においてイオンの形態から析出し、セメント粒子間に分散した状態で脱水処理するため、脱水ケーキ中においても均一に分散した状態で硬化し、より安定化できる。
【０００８】
【実施例】
以下、実施例で本発明を説明する。尚、６価クロム濃度はジフェニルカルバジッド吸光光度法、全クロムについては過マンガン酸カリウムで６価クロムに酸化した後、ジフェニルカルバジッド吸光光度法で定量測定した。
（試験例１） 重クロム酸カリウムで作成した濃度２ｐｐｍの６価クロム溶液を廃水モデルとして供試した。アクリル板製のL160×W120×H120mmの電気還元槽に前記廃水モデル１０００ｍｌを入れ、これに普通ポルトランドセメント５０ｇを添加撹拌し懸濁液とした。尚、普通ポルトランドセメント添加後の懸濁液の全クロム濃度は２．３２ｐｐｍ、６価クロム濃度は２．３１ｐｐｍとなり、セメントからの溶出のためクロム濃度は少し上昇していた。この懸濁液について陽極及び陰極がそれぞれ厚み３ｍｍのアルミニウム板（浸漬面積：80×50mm）から構成され、電極間隔５cmの電極を用い、マグネットスターラーで撹拌しながら、印加電圧６０V、処理時間３０分の電気還元を実施した。続いて電気還元した懸濁液をろ紙（ワットマンNo2、Φ150mm）を用い吸引ろ過して固液分離した後、ろ過水中の全クロム及び６価クロム濃度を測定した。また、普通ポルトランドセメントを添加せずに廃水モデルに対して同様に電気還元を実施し、吸引ろ過（ワットマンNo2、Φ150mm）した際のろ過水中の全クロム及び６価クロム濃度を測定したものを比較例１とした。その結果を表１に示す。尚、セメントを添加しなかった廃水モデルは電気伝導度が低く電気還元に問題があったため、電解質として少量の塩化ナトリウムを添加し試験を行った。
【０００９】
【表１】
表１に示されるように、試験例１の６価クロムを含む廃水をセメントの添加でアルカリ性の懸濁液として電気還元した場合は、３価クロムは不溶性化合物となりセメントと共に固液分離されるために、ろ過水中のクロムは検出限界以下になった。一方、比較例１のセメントを添加しないで電気還元した場合もろ過水中の６価クロムは検出限界以下となったが、全クロムについては検出された。尚、試験例１で固液分離後にろ紙上に残った固形分はセメントの硬化によって硬質物になった。本発明を実施することで、６価クロムを３価クロムに還元でき、更にセメント中に封じ込めることが可能であることが分かった。
【００１１】
（試験例２） 重クロム酸カリウムで作成した濃度４０ｐｐｍの６価クロムを含む土壌を作成しモデルとして供試した。アクリル板製のL160×W120×H120mmの電気還元槽に水１０００ｍｌ及び前記土壌モデル５０ｇを添加し６価クロムを溶出させた。この溶出液について普通ポルトランドセメントを３ｇ、５ｇ、１０ｇあるいは２０ｇ添加し、セメントの添加量が異なる懸濁液を調製した。この懸濁液についてそれぞれ水素イオン濃度、全クロム及び６価クロム濃度を測定した。続いて試験例１で示した条件で電気還元を実施した後、ろ紙（ワットマンNo2、Φ150mm）を用い吸引ろ過で固液分離して得られたろ過水中の全クロム及び６価クロム濃度を測定した。その結果を表２に示す。
【００１２】
【表２】
表２に示されるように試験例２の土壌モデルの場合、セメントの添加量が３ｇの時に水素イオン濃度はｐＨ８．３であった。このｐＨ８．３の水素イオン濃度において電気還元を実施した場合、ろ過水中の６価クロムは検出限界以下となったが、全クロムについては検出されており、３価クロムの不溶化が不十分であった。一方、水素イオン濃度がｐＨ９．９以上の場合はろ過水中にクロムは検出されなかった。
【００１４】
（試験例３） アクリル板製のL160×W120×H120mmの電気還元槽に水１０００ｍｌ及び試験例２で作成した土壌モデル５０ｇを添加し６価クロムを溶出させた。この溶出液について普通ポルトランドセメント５０ｇを添加し懸濁液を調製した。尚、懸濁液の全クロム濃度は２．４２ｐｐｍ、６価クロム濃度は２．３９ｐｐｍであった。この懸濁液についてアルミニウム、鉄、チタン、銅及びマグネシウム合金の各電極材質を用い、試験例１で示した条件で電気還元した際のろ過水中の全クロム及び６価クロム濃度を測定した。その結果を表３に示す。
【００１５】
【表３】
（試験例４） アクリル板製のL160×W120×H120mmの電気還元槽にミキサー車洗浄過程で排出されたセメントを含むスラッジ水（固形分濃度７％）１０００ｍｌを入れ、これに試験例２で作成した土壌モデル５０ｇを添加撹拌しアルカリ性懸濁液とした。この懸濁液の全クロム濃度は２．７６ｐｐｍ、６価クロム濃度は２．６９ｐｐｍであった。このアルカリ性の懸濁液についてアルミニウム及び鉄の各電極材質を用い、試験例１と同様に電気還元した際のろ過水中の全クロム及び６価クロム濃度を測定した。また、固液分離後にろ紙上に残った固形物を３０日間養生固化して得た硬質物中からの全クロム及び６価クロムの溶出量を測定した。こその結果を表４に示す。
【００１７】
【表４】
表４に示されるように、ろ過水中のクロムは検出限界以下であり、ミキサー車洗浄過程で排出されたスラッジ水中のセメントが利用できることが分かった。また、固液分離後のろ紙上に残った硬質物からのクロムの溶出も殆どなく、クロムはセメントによって封じ込められた。
【００１９】
（試験例５） 農業用ため池から採取したクロムを含まない浚渫土スラリー（含水率９２％）１ｋｇに普通ポルトランドセメント５０ｇを添加撹拌し懸濁液を作成した。このアルカリ性の懸濁液の全クロム濃度は０．４１ｐｐｍ、６価クロム濃度は０．３７ｐｐｍであった。この懸濁液について試験例１と同様に印加電圧６０V、処理時間３０分の電気還元を実施した。続いて電気還元した懸濁液をろ紙（ワットマンNo2、Φ150mm）を用い吸引ろ過して固液分離した後、ろ過水中の全クロム及び６価クロム濃度を測定した。また、固液分離で得られた固形分（改良土）を３０日間養生し硬化させた後、改良土から溶出する全クロム及び６価クロム濃度を測定した。尚、前記懸濁液を電気還元せずに固液分離したものについてろ過水中の全クロム及び６価クロム濃度を測定し、また、固形分（改良土）を３０日間養生し硬化させた後の改良土から溶出する全クロム及び６価クロム濃度を測定したものを比較例５とした。その結果を表５に示す。
【００２０】
【表５】
表５の結果から分かるように、試験例５ではろ過水中の６価クロムは検出限界以下であり、また、改良土からの６価クロムも検出限界以下であった。一方、電気還元を実施していない比較例５では還元による無毒化がないためにろ過水中に６価クロムが検出され、また、改良土からも僅かに６価クロムの溶出が確認された。以上のことから本発明を浚渫土スラリーにセメントを添加しフィルタープレス脱水で改良土を製造する様な土木工法に応用することで、浚渫土及びセメントから溶出する６価クロムを無毒化し、更にろ過水中に出さないため、ろ過水は浚渫土が採取された水域へｐH調製だけで戻せるようになる。また、改良土には６価クロムの状態ではなく３価クロムの状態で封じ込められるため、６価クロムの再溶出の問題が改善できる。
【００２２】
【発明の効果】
請求項１によれば、対象物中の６価クロムを無毒化するだけでなく、セメント由来の６価クロムをも無毒化処理し、簡便に硬質状態の固形物中に封じ込めることができる。
請求項２によれば、セメントから供給させるアルカリ分で効率的に３価クロムから水酸化クロムへ変換でき、水酸化ナトリウム等を供給する必要がない。また、電気伝導度が低い対象物中の６価クロムを処理する場合でもセメントのアルカリ分が電解質として働くため効率的に電気還元できるという効果を併せ持つ。
請求項３によれば、前記アルカリ性の懸濁液の調製に生コンクリートの混練設備や残存生コンクリートの処理工程から排出される硬化能力を持つセメントを含んだスラッジ水を利用することで、廃棄物となるスラッジ水の有効利用が図れるだけでなく、６価クロムの無毒化処理がより経済的に実施できる。
本発明は、セメントに含まれる６価クロムの無毒化も同時に行うため、セメント系廃水の６価クロム除去だけでなく、セメントを用いた重金属の安定化処理等において６価クロムの副次的な汚染を防止する手段となる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for detoxifying hexavalent chromium contained in an object such as waste water, incineration ash, sludge and soil or cement used as a solidifying material by electroreduction.
[0002]
[Prior art]
As a technique for preventing elution of heavy metals and chemical substances contained in waste water, incinerated ash, sludge, soil, etc. into the environment or re-mudging of various sludge cakes, there is a method of solidifying with cement and stabilizing. On the other hand, among heavy metals, hexavalent chromium has a risk of causing skin ulcer, liver damage, taste and smell sense, and is designated as a hazardous substance by the Water Pollution Control Law. Although this hexavalent chromium is contained in the cement in a small amount, when the improved soil is produced from the clay by adding the cement, an elution test for hexavalent chromium from the improved soil is required. Generally, hexavalent chromium is detoxified by a method of converting hexavalent chromium to trivalent chromium by a reducing agent or electroreduction, a step of converting chromium hydroxide into a precipitate with sodium hydroxide or the like, a dehydration treatment and a precipitate. There are those that are stabilized through a step of discharging as a dehydrated cake and a step of hardening with cement or the like to bring them into a stable disposal site.
[0003]
[Problems to be solved by the invention]
In the method of solidifying and stabilizing with cement, since hexavalent chromium originally contained in the cement is not treated at all, it is difficult to say that it is completely detoxified and stabilized. In particular, in the detoxification method for hexavalent chromium contained in objects such as waste water, incinerated ash, sludge, and soil, dehydration discharged after the step of converting to trivalent chromium by a reducing agent or electroreduction. In the procedure of hardening the cake with cement, hexavalent chromium derived from cement is brought back into the dehydrated cake detoxified as trivalent chromium. Further, when the cement is added and stirred in the precipitation tank after the reduction step, and chromium hydroxide is dehydrated with the cement by a dehydrator and discharged as a cake, hexavalent chromium contained in the cement is eluted to the filtered water side. I also have the problem.
The present invention has been made in order to solve these problems. In addition to detoxifying hexavalent chromium in an object, the present invention not only detoxifies cement-derived hexavalent chromium, but can be easily hardened. The object is to provide a method for containment in solids.
[0004]
[Means for Solving the Problems]
As a result of diligent examination in view of the above circumstances, the object can be electroreduced from trivalent chromium to trivalent chromium in a suspension state by adding cement having a curing ability, and further under alkaline conditions caused by the cement. Thus, the inventors have found that trivalent chromium is immediately changed into an insoluble compound of chromium hydroxide, and thus can be dehydrated and removed in a state where it is entrained in cement, thereby completing the present invention.
That is, the present invention is a method of treating hexavalent chromium contained in a target object such as waste water, incinerated ash, sludge, and soil or cement used as a solidifying material, and adding a hardenable cement to the target object. Electroreduction of trivalent chromium in a mixing step for producing a turbid liquid and hexavalent chromium dissolved in the suspension obtained from the mixing step in an electric reduction tank comprising at least a pair of electrodes composed of an anode and a cathode A detoxification step, and a hexavalent chromium treatment method comprising a dehydration process in which trivalent chromium obtained from the detoxification step is dehydrated together with cement in the form of chromium hydroxide and treated as a dehydrated cake.
The suspension is characterized in that the hydrogen ion concentration is pH 9 or more.
Further, the suspension is prepared by using sludge water containing cement having a curing ability discharged from a ready concrete mixing facility or a residual ready concrete processing step.
[0005]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is based on the fact that the detoxification treatment by electroreduction of hexavalent chromium is carried out in the state of a suspension by adding cement having a curing ability. This cement not only has the effect of forming a solid solid by a curing reaction, but also supplies alkali to convert trivalent chromium produced by the reduction reaction into chromium hydroxide, which is an insoluble compound, and also efficiently performs electroreduction. It has the effect | action which supplies the electrolyte for implementation, and this invention utilizes these effects actively and effectively.
The object containing hexavalent chromium according to the present invention may be in a solution state such as plating waste water or electrolytic processing waste water, or in a solid state state such as incineration ash or soil. In addition, waste water containing hexavalent chromium derived from cement, which is used as a hardener in the present invention, such as sludge water containing cement discharged from a raw concrete kneading facility or a residual raw concrete treatment process, is also an object.
[0006]
In the mixing step of the present invention, before subjecting hexavalent chromium contained in the object to electroreduction, cement is added to the object in advance to form a suspension. At this time, the amount of cement added is adjusted so that the hydrogen ion concentration of the suspension is alkaline with a pH of 9 or more. In addition, the object and the cement can be made into a suspension in a separately installed mixing and stirring tank, or can be mixed into a suspension in an electric reduction tank. Furthermore, when using sludge water containing cement with a curing ability discharged from the ready-mixed concrete mixing facility or the remaining ready-mixed concrete processing process, the cement can be easily added by stirring the target object to the sludge water. A suspension to be added can be prepared.
In the detoxification process of hexavalent chromium according to the present invention, hexavalent chromium is treated together with the cement and other impurities in the object. Since the cement is self-hardening, the inner wall of the electroreduction tank, the electrode In order to prevent sticking to etc., the suspension is allowed to flow without stagnation. As a means for flowing, it may be circulated by a pump or stirred by a stirring blade, and these may be used in combination. After the treatment, measures are taken to remove the deposits on the inner wall and electrode of the electric reduction tank. Examples of the means for removing the deposits include washing with a sprinkler or immersion with fresh water injection. In addition, in electroreduction, precipitates are generated on the electrode and the electric resistance is likely to increase. Therefore, it is preferable to perform polarity reversal control as needed to clean the electrode.
[0007]
The electrode to be used is not limited in material, shape and size, and is selected in consideration of the effect, cost and use conditions. Examples of the material of the electrode include platinum, aluminum, iron, stainless steel, titanium, magnesium alloy, zinc, copper, and other metal-based materials, carbon rods, and the like. The same material may be used for the anode and the cathode as a pair of electrodes. Different materials may be combined and used as a pair of electrodes. In particular, iron is not only easy to obtain, but when used as an anode, it is a suitable material because it produces divalent iron ions with a strong reducing action and has a high function to supplement the reduction of hexavalent chromium. . Further, the amount of electricity at the time of electroreduction may be determined based on the hexavalent chromium concentration in the object, the treatment time, and the like, and usually a value larger than the theoretical value obtained from Faraday's law is used.
In the detoxification step of the present invention, hexavalent chromium is reduced to trivalent chromium, and in the wastewater treatment step, trivalent chromium is dehydrated together with cement in the form of chromium hydroxide and treated as a dehydrated cake. As a device for performing dehydration, a filter press machine, a centrifugal dehydrator, a belt press machine, or the like can be used. The dehydrated cake is cured by curing for a certain period of time, and finally chromium hydroxide is contained in a stable solid.
Chromium hydroxide generated from trivalent chromium precipitates from the form of ions in the suspension containing cement, and is dehydrated in a state of being dispersed between cement particles, so that it is uniformly dispersed in the dehydrated cake. It can be cured and more stabilized.
[0008]
【Example】
Hereinafter, the present invention will be described by way of examples. The hexavalent chromium concentration was quantitatively measured by diphenylcarbazide absorptiometry, and the total chromium was quantitatively measured by diphenylcarbazide absorptiometry after oxidation to hexavalent chromium with potassium permanganate.
(Test Example 1) A hexavalent chromium solution having a concentration of 2 ppm prepared with potassium dichromate was used as a wastewater model. 1000 ml of the waste water model was placed in an L160 × W120 × H120 mm electroreduction tank made of an acrylic plate, and 50 g of ordinary Portland cement was added and stirred into this suspension. The total chromium concentration of the suspension after addition of ordinary Portland cement was 2.32 ppm, and the hexavalent chromium concentration was 2.31 ppm. The chromium concentration was slightly increased due to elution from the cement. For this suspension, the anode and cathode are each composed of an aluminum plate (immersion area: 80 × 50 mm) with a thickness of 3 mm. Using an electrode with an electrode spacing of 5 cm, stirring with a magnetic stirrer, applied voltage 60 V, treatment time 30 minutes Was carried out. Subsequently, the electroreduced suspension was subjected to suction filtration using a filter paper (Whatman No. 2, Φ150 mm) for solid-liquid separation, and then the total chromium and hexavalent chromium concentrations in the filtered water were measured. In addition, the same electroreduction was carried out on the wastewater model without adding ordinary Portland cement, and the total chromium and hexavalent chromium concentrations measured in the filtered water after suction filtration (Whatman No2, Φ150mm) were compared. Example 1 was adopted. The results are shown in Table 1. In addition, since the waste water model which did not add cement had a problem in electroreduction with low electrical conductivity, a small amount of sodium chloride was added as an electrolyte and the test was performed.
[0009]
[Table 1]
As shown in Table 1, when waste water containing hexavalent chromium of Test Example 1 is electroreduced as an alkaline suspension by adding cement, trivalent chromium becomes an insoluble compound and is solid-liquid separated together with cement. In addition, chromium in the filtered water was below the detection limit. On the other hand, even when electroreduction was performed without adding the cement of Comparative Example 1, hexavalent chromium in the filtered water was below the detection limit, but all chromium was detected. In Test Example 1, the solid content remaining on the filter paper after the solid-liquid separation became a hard material due to the hardening of the cement. By carrying out the present invention, it has been found that hexavalent chromium can be reduced to trivalent chromium and can be further encapsulated in cement.
[0011]
(Test Example 2) A soil containing hexavalent chromium having a concentration of 40 ppm prepared with potassium dichromate was prepared and used as a model. 1000 ml of water and 50 g of the soil model were added to an L160 × W120 × H120 mm electric reduction tank made of an acrylic plate to elute hexavalent chromium. To this eluate, 3 g, 5 g, 10 g or 20 g of ordinary Portland cement was added to prepare suspensions with different amounts of cement. The hydrogen ion concentration, total chromium, and hexavalent chromium concentration were measured for each suspension. Subsequently, after carrying out the electroreduction under the conditions shown in Test Example 1, the total chromium and hexavalent chromium concentrations in the filtered water obtained by solid-liquid separation by suction filtration using filter paper (Whatman No2, Φ150 mm) were measured. . The results are shown in Table 2.
[0012]
[Table 2]
As shown in Table 2, in the case of the soil model of Test Example 2, the hydrogen ion concentration was pH 8.3 when the added amount of cement was 3 g. When electroreduction was carried out at this hydrogen ion concentration of pH 8.3, hexavalent chromium in filtered water was below the detection limit, but all chromium was detected and insolubilization of trivalent chromium was insufficient. It was. On the other hand, when the hydrogen ion concentration was pH 9.9 or higher, chromium was not detected in the filtered water.
[0014]
(Test Example 3) 1000 ml of water and 50 g of the soil model prepared in Test Example 2 were added to an L160 × W120 × H120 mm electric reduction tank made of an acrylic plate to elute hexavalent chromium. To this eluate, 50 g of ordinary Portland cement was added to prepare a suspension. The total chromium concentration of the suspension was 2.42 ppm, and the hexavalent chromium concentration was 2.39 ppm. This suspension was measured for the total chromium and hexavalent chromium concentrations in the filtered water when each electrode material of aluminum, iron, titanium, copper and magnesium alloy was electroreduced under the conditions shown in Test Example 1. The results are shown in Table 3.
[0015]
[Table 3]
(Test Example 4) 1000 ml of sludge water (solid content concentration 7%) containing cement discharged in the mixer truck washing process was placed in an L160 x W120 x H120 mm electric reduction tank made of an acrylic plate. 50 g of the soil model was added and stirred to obtain an alkaline suspension. This suspension had a total chromium concentration of 2.76 ppm and a hexavalent chromium concentration of 2.69 ppm. This alkaline suspension was measured for the total chromium and hexavalent chromium concentrations in the filtrate when electroreduced in the same manner as in Test Example 1, using aluminum and iron electrode materials. Moreover, the elution amount of total chromium and hexavalent chromium from the hard material obtained by curing and solidifying the solid material remaining on the filter paper for 30 days after solid-liquid separation was measured. The results are shown in Table 4.
[0017]
[Table 4]
As shown in Table 4, it was found that the chromium in the filtered water is below the detection limit, and the cement in the sludge water discharged during the mixer truck washing process can be used. Moreover, there was almost no elution of chromium from the hard material remaining on the filter paper after the solid-liquid separation, and the chromium was contained by the cement.
[0019]
(Test Example 5) 50 g of ordinary Portland cement was added to 1 kg of a clay slurry containing no chromium (water content: 92%) collected from an agricultural pond to prepare a suspension. This alkaline suspension had a total chromium concentration of 0.41 ppm and a hexavalent chromium concentration of 0.37 ppm. This suspension was electroreduced in the same manner as in Test Example 1 with an applied voltage of 60 V and a treatment time of 30 minutes. Subsequently, the electroreduced suspension was subjected to suction filtration using a filter paper (Whatman No. 2, Φ150 mm) for solid-liquid separation, and then the total chromium and hexavalent chromium concentrations in the filtered water were measured. Further, after solidifying (improving soil) obtained by solid-liquid separation for 30 days and curing, the total chromium and hexavalent chromium concentrations eluted from the improved soil were measured. In addition, after solid-liquid separation of the suspension without electroreduction, the total chromium and hexavalent chromium concentrations in the filtered water were measured, and the solid content (improved soil) was cured and cured for 30 days. Comparative Example 5 was obtained by measuring the total chromium and hexavalent chromium concentrations eluted from the improved soil. The results are shown in Table 5.
[0020]
[Table 5]
As can be seen from the results of Table 5, in Test Example 5, hexavalent chromium in the filtered water was below the detection limit, and hexavalent chromium from the improved soil was also below the detection limit. On the other hand, in Comparative Example 5 in which electroreduction was not performed, hexavalent chromium was detected in the filtered water because there was no detoxification by reduction, and elution of hexavalent chromium was also confirmed from the improved soil. From the above, the present invention is applied to a civil engineering method in which cement is added to the clay slurry and improved soil is produced by filter press dehydration, thereby detoxifying hexavalent chromium eluted from the clay and cement and further filtering. Since it is not discharged into the water, the filtered water can be returned to the water area from which the dredged material has been collected only by pH adjustment. Further, since the improved soil is contained not in the hexavalent chromium state but in the trivalent chromium state, the problem of re-elution of the hexavalent chromium can be improved.
[0022]
【The invention's effect】
According to the first aspect, not only the hexavalent chromium in the object is detoxified, but also the hexavalent chromium derived from the cement can be detoxified and easily contained in a hard solid.
According to claim 2, it is possible to efficiently convert trivalent chromium to chromium hydroxide with an alkali component supplied from cement, and there is no need to supply sodium hydroxide or the like. Further, even when hexavalent chromium in an object having low electrical conductivity is processed, the alkaline content of the cement works as an electrolyte, so that it can be efficiently electroreduced.
According to claim 3, waste is produced by using sludge water containing cement having a curing ability discharged from a raw concrete kneading facility or a residual raw concrete processing step in preparation of the alkaline suspension. Not only can sludge water be effectively used, hexavalent chromium can be detoxified more economically.
In the present invention, hexavalent chromium contained in cement is also detoxified at the same time. Therefore, in addition to removal of hexavalent chromium in cement-based wastewater, secondary treatment of hexavalent chromium is not only used in heavy metal stabilization treatment using cement. It is a means to prevent contamination.
Claims (3)
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